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Molarity

By the end of this section, you will be able to:

In preceding sections, we focused on the composition of substances: samples of matter that contain only one type of element or compound. However, mixtures—samples of matter containing two or more substances physically combined—are more commonly encountered in nature than are pure substances. Similar to a pure substance, the relative composition of a mixture plays an important role in determining its properties. The relative amount of oxygen in a planet’s atmosphere determines its ability to sustain aerobic life. The relative amounts of iron, carbon, nickel, and other elements in steel (a mixture known as an “alloy”) determine its physical strength and resistance to corrosion. The relative amount of the active ingredient in a medicine determines its effectiveness in achieving the desired pharmacological effect. The relative amount of sugar in a beverage determines its sweetness (see [link]). In this section, we will describe one of the most common ways in which the relative compositions of mixtures may be quantified.

Sugar is one of many components in the complex mixture known as coffee. The amount of sugar in a given amount of coffee is an important determinant of the beverage’s sweetness. (credit: Jane Whitney)
A picture is shown of sugar being poured from a spoon into a cup.

Solutions

We have previously defined solutions as homogeneous mixtures, meaning that the composition of the mixture (and therefore its properties) is uniform throughout its entire volume. Solutions occur frequently in nature and have also been implemented in many forms of manmade technology. We will explore a more thorough treatment of solution properties in the chapter on solutions and colloids, but here we will introduce some of the basic properties of solutions.

The relative amount of a given solution component is known as its concentration. Often, though not always, a solution contains one component with a concentration that is significantly greater than that of all other components. This component is called the solvent and may be viewed as the medium in which the other components are dispersed, or dissolved. Solutions in which water is the solvent are, of course, very common on our planet. A solution in which water is the solvent is called an aqueous solution.

A solute is a component of a solution that is typically present at a much lower concentration than the solvent. Solute concentrations are often described with qualitative terms such as dilute (of relatively low concentration) and concentrated (of relatively high concentration).

Concentrations may be quantitatively assessed using a wide variety of measurement units, each convenient for particular applications. Molarity (M) is a useful concentration unit for many applications in chemistry. Molarity is defined as the number of moles of solute in exactly 1 liter (1 L) of the solution:

M=mol soluteL solutionM=mol soluteL solution

Calculating Molar Concentrations A 355-mL soft drink sample contains 0.133 mol of sucrose (table sugar). What is the molar concentration of sucrose in the beverage?

Solution Since the molar amount of solute and the volume of solution are both given, the molarity can be calculated using the definition of molarity. Per this definition, the solution volume must be converted from mL to L:

M=mol soluteL solution=0.133mol355mL×1L1000mL=0.375MM=mol soluteL solution=0.133mol355mL×1L1000mL=0.375M

Check Your Learning A teaspoon of table sugar contains about 0.01 mol sucrose. What is the molarity of sucrose if a teaspoon of sugar has been dissolved in a cup of tea with a volume of 200 mL?

Answer:

0.05 M

Deriving Moles and Volumes from Molar Concentrations How much sugar (mol) is contained in a modest sip (~10 mL) of the soft drink from [link]?

Solution In this case, we can rearrange the definition of molarity to isolate the quantity sought, moles of sugar. We then substitute the value for molarity that we derived in [link], 0.375 M:

M=mol soluteL solutionmol solute=M×L solutionmol solute=0.375mol sugarL×(10mL×1L1000mL)=0.004mol sugarM=mol soluteL solutionmol solute=M×L solutionmol solute=0.375mol sugarL×(10mL×1L1000mL)=0.004mol sugar

Check Your Learning What volume (mL) of the sweetened tea described in [link] contains the same amount of sugar (mol) as 10 mL of the soft drink in this example?

Answer:

80 mL

Calculating Molar Concentrations from the Mass of Solute Distilled white vinegar ([link]) is a solution of acetic acid, CH3CO2H, in water. A 0.500-L vinegar solution contains 25.2 g of acetic acid. What is the concentration of the acetic acid solution in units of molarity?

Distilled white vinegar is a solution of acetic acid in water.
A label on a container is shown. The label has a picture of a salad with the words “Distilled White Vinegar,” and, “Reduced with water to 5% acidity,” written above it.

Solution As in previous examples, the definition of molarity is the primary equation used to calculate the quantity sought. In this case, the mass of solute is provided instead of its molar amount, so we must use the solute’s molar mass to obtain the amount of solute in moles:

M=mol soluteL solution=25.2 gCH3CO2H×1mol CH2CO2H60.052 g CH2CO2H0.500 L solution=0.839MM=mol soluteL solution=25.2 gCH3CO2H×1mol CH2CO2H60.052 g CH2CO2H0.500 L solution=0.839M
M=mol soluteL solution=0.839MM=0.839mol solute1.00L solutionM=mol soluteL solution=0.839MM=0.839mol solute1.00L solution

Check Your Learning Calculate the molarity of 6.52 g of CoCl2 (128.9 g/mol) dissolved in an aqueous solution with a total volume of 75.0 mL.

Answer:

0.674 M

Determining the Mass of Solute in a Given Volume of Solution How many grams of NaCl are contained in 0.250 L of a 5.30-M solution?

Solution The volume and molarity of the solution are specified, so the amount (mol) of solute is easily computed as demonstrated in [link]:

M=mol soluteL solutionmol solute=M×L solutionmol solute=5.30mol NaClL×0.250L=1.325mol NaClM=mol soluteL solutionmol solute=M×L solutionmol solute=5.30mol NaClL×0.250L=1.325mol NaCl

Finally, this molar amount is used to derive the mass of NaCl:

1.325 mol NaCl×58.44g NaClmol NaCl=77.4g NaCl1.325 mol NaCl×58.44g NaClmol NaCl=77.4g NaCl

Check Your Learning How many grams of CaCl2 (110.98 g/mol) are contained in 250.0 mL of a 0.200-M solution of calcium chloride?

Answer:

5.55 g CaCl2

When performing calculations stepwise, as in [link], it is important to refrain from rounding any intermediate calculation results, which can lead to rounding errors in the final result. In [link], the molar amount of NaCl computed in the first step, 1.325 mol, would be properly rounded to 1.32 mol if it were to be reported; however, although the last digit (5) is not significant, it must be retained as a guard digit in the intermediate calculation. If we had not retained this guard digit, the final calculation for the mass of NaCl would have been 77.1 g, a difference of 0.3 g.

In addition to retaining a guard digit for intermediate calculations, we can also avoid rounding errors by performing computations in a single step (see [link]). This eliminates intermediate steps so that only the final result is rounded.

Determining the Volume of Solution Containing a Given Mass of Solute In [link], we found the typical concentration of vinegar to be 0.839 M. What volume of vinegar contains 75.6 g of acetic acid?

Solution First, use the molar mass to calculate moles of acetic acid from the given mass:

g solute×mol soluteg solute=mol soluteg solute×mol soluteg solute=mol solute

Then, use the molarity of the solution to calculate the volume of solution containing this molar amount of solute:

mol solute×L solutionmol solute=L solutionmol solute×L solutionmol solute=L solution

Combining these two steps into one yields:

g solute×mol soluteg solute×L solutionmol solute=L solutiong solute×mol soluteg solute×L solutionmol solute=L solution
75.6gCH3CO2H(molCH3CO2H60.05g)(L solution0.839molCH3CO2H)=1.50L solution75.6gCH3CO2H(molCH3CO2H60.05g)(L solution0.839molCH3CO2H)=1.50L solution

Check Your Learning What volume of a 1.50-M KBr solution contains 66.0 g KBr?

Answer:

0.370 L

Dilution of Solutions

Dilution is the process whereby the concentration of a solution is lessened by the addition of solvent. For example, we might say that a glass of iced tea becomes increasingly diluted as the ice melts. The water from the melting ice increases the volume of the solvent (water) and the overall volume of the solution (iced tea), thereby reducing the relative concentrations of the solutes that give the beverage its taste ([link]).

Both solutions contain the same mass of copper nitrate. The solution on the right is more dilute because the copper nitrate is dissolved in more solvent. (credit: Mark Ott)
This figure shows two graduated cylinders side-by-side. The first has about half as much blue liquid as the second. The blue liquid is darker in the first cylinder than in the second.

Dilution is also a common means of preparing solutions of a desired concentration. By adding solvent to a measured portion of a more concentrated stock solution, we can achieve a particular concentration. For example, commercial pesticides are typically sold as solutions in which the active ingredients are far more concentrated than is appropriate for their application. Before they can be used on crops, the pesticides must be diluted. This is also a very common practice for the preparation of a number of common laboratory reagents ([link]).

A solution of KMnO4 is prepared by mixing water with 4.74 g of KMnO4 in a flask. (credit: modification of work by Mark Ott)
This figure shows two photos. In the first, there is an empty glass container, 4.75 g of K M n O subscript 4 powder on a white circle, and a bottle of distilled water. In the second photo the powder and about half the water have been added to the glass container. The liquid in the glass container is almost black in color.

A simple mathematical relationship can be used to relate the volumes and concentrations of a solution before and after the dilution process. According to the definition of molarity, the molar amount of solute in a solution is equal to the product of the solution’s molarity and its volume in liters:

n=MLn=ML

Expressions like these may be written for a solution before and after it is diluted:

n1=M1L1n1=M1L1
n2=M2L2n2=M2L2

where the subscripts “1” and “2” refer to the solution before and after the dilution, respectively. Since the dilution process does not change the amount of solute in the solution,n1 = n2. Thus, these two equations may be set equal to one another:

M1L1=M2L2M1L1=M2L2

This relation is commonly referred to as the dilution equation. Although we derived this equation using molarity as the unit of concentration and liters as the unit of volume, other units of concentration and volume may be used, so long as the units properly cancel per the factor-label method. Reflecting this versatility, the dilution equation is often written in the more general form:

C1V1=C2V2C1V1=C2V2

where C and V are concentration and volume, respectively.

Determining the Concentration of a Diluted Solution If 0.850 L of a 5.00-M solution of copper nitrate, Cu(NO3)2, is diluted to a volume of 1.80 L by the addition of water, what is the molarity of the diluted solution?

Solution We are given the volume and concentration of a stock solution, V1 and C1, and the volume of the resultant diluted solution, V2. We need to find the concentration of the diluted solution, C2. We thus rearrange the dilution equation in order to isolate C2:

C1V1=C2V2C2=C1V1V2C1V1=C2V2C2=C1V1V2

Since the stock solution is being diluted by more than two-fold (volume is increased from 0.85 L to 1.80 L), we would expect the diluted solution’s concentration to be less than one-half 5 M. We will compare this ballpark estimate to the calculated result to check for any gross errors in computation (for example, such as an improper substitution of the given quantities). Substituting the given values for the terms on the right side of this equation yields:

C2=0.850L×5.00molL1.80 L=2.36MC2=0.850L×5.00molL1.80 L=2.36M

This result compares well to our ballpark estimate (it’s a bit less than one-half the stock concentration, 5 M).

Check Your Learning What is the concentration of the solution that results from diluting 25.0 mL of a 2.04-M solution of CH3OH to 500.0 mL?

Answer:

0.102 M CH3OH

Volume of a Diluted Solution What volume of 0.12 M HBr can be prepared from 11 mL (0.011 L) of 0.45 M HBr?

Solution We are given the volume and concentration of a stock solution, V1 and C1, and the concentration of the resultant diluted solution, C2. We need to find the volume of the diluted solution, V2. We thus rearrange the dilution equation in order to isolate V2:

C1V1=C2V2V2=C1V1C2C1V1=C2V2V2=C1V1C2

Since the diluted concentration (0.12 M) is slightly more than one-fourth the original concentration (0.45 M), we would expect the volume of the diluted solution to be roughly four times the original volume, or around 44 mL. Substituting the given values and solving for the unknown volume yields:

V2=(0.45M)(0.011L)(0.12M)V2=0.041LV2=(0.45M)(0.011L)(0.12M)V2=0.041L

The volume of the 0.12-M solution is 0.041 L (41 mL). The result is reasonable and compares well with our rough estimate.

Check Your Learning A laboratory experiment calls for 0.125 M HNO3. What volume of 0.125 M HNO3 can be prepared from 0.250 L of 1.88 M HNO3?

Answer:

3.76 L

Volume of a Concentrated Solution Needed for Dilution What volume of 1.59 M KOH is required to prepare 5.00 L of 0.100 M KOH?

Solution We are given the concentration of a stock solution, C1, and the volume and concentration of the resultant diluted solution, V2 and C2. We need to find the volume of the stock solution, V1. We thus rearrange the dilution equation in order to isolate V1:

C1V1=C2V2V1=C2V2C1C1V1=C2V2V1=C2V2C1

Since the concentration of the diluted solution 0.100 M is roughly one-sixteenth that of the stock solution (1.59 M), we would expect the volume of the stock solution to be about one-sixteenth that of the diluted solution, or around 0.3 liters. Substituting the given values and solving for the unknown volume yields:

V1=(0.100M)(5.00L)1.59MV1=0.314LV1=(0.100M)(5.00L)1.59MV1=0.314L

Thus, we would need 0.314 L of the 1.59-M solution to prepare the desired solution. This result is consistent with our rough estimate.

Check Your Learning What volume of a 0.575-M solution of glucose, C6H12O6, can be prepared from 50.00 mL of a 3.00-M glucose solution?

Answer:

0.261 L

Key Concepts and Summary

Solutions are homogeneous mixtures. Many solutions contain one component, called the solvent, in which other components, called solutes, are dissolved. An aqueous solution is one for which the solvent is water. The concentration of a solution is a measure of the relative amount of solute in a given amount of solution. Concentrations may be measured using various units, with one very useful unit being molarity, defined as the number of moles of solute per liter of solution. The solute concentration of a solution may be decreased by adding solvent, a process referred to as dilution. The dilution equation is a simple relation between concentrations and volumes of a solution before and after dilution.

Chemistry End of Chapter Exercises

Explain what changes and what stays the same when 1.00 L of a solution of NaCl is diluted to 1.80 L.

What information do we need to calculate the molarity of a sulfuric acid solution?

We need to know the number of moles of sulfuric acid dissolved in the solution and the volume of the solution.

What does it mean when we say that a 200-mL sample and a 400-mL sample of a solution of salt have the same molarity? In what ways are the two samples identical? In what ways are these two samples different?

Determine the molarity for each of the following solutions:

(a) 0.444 mol of CoCl2 in 0.654 L of solution

(b) 98.0 g of phosphoric acid, H3PO4, in 1.00 L of solution

(c) 0.2074 g of calcium hydroxide, Ca(OH)2, in 40.00 mL of solution

(d) 10.5 kg of Na2SO4·10H2O in 18.60 L of solution

(e) 7.0 ×× 10−3 mol of I2 in 100.0 mL of solution

(f) 1.8 ×× 104 mg of HCl in 0.075 L of solution

(a) 0.679 M;


(b) 1.00 M;

(c) 0.06998 M;

(d) 1.75 M;

(e) 0.070 M;

(f) 6.6 M

Determine the molarity of each of the following solutions:

(a) 1.457 mol KCl in 1.500 L of solution

(b) 0.515 g of H2SO4 in 1.00 L of solution

(c) 20.54 g of Al(NO3)3 in 1575 mL of solution

(d) 2.76 kg of CuSO4·5H2O in 1.45 L of solution

(e) 0.005653 mol of Br2 in 10.00 mL of solution

(f) 0.000889 g of glycine, C2H5NO2, in 1.05 mL of solution

Consider this question: What is the mass of the solute in 0.500 L of 0.30 M glucose, C6H12O6, used for intravenous injection?

(a) Outline the steps necessary to answer the question.

(b) Answer the question.

(a) determine the number of moles of glucose in 0.500 L of solution; determine the molar mass of glucose; determine the mass of glucose from the number of moles and its molar mass; (b) 27 g

Consider this question: What is the mass of solute in 200.0 L of a 1.556-M solution of KBr?

(a) Outline the steps necessary to answer the question.

(b) Answer the question.

Calculate the number of moles and the mass of the solute in each of the following solutions:

(a) 2.00 L of 18.5 M H2SO4, concentrated sulfuric acid

(b) 100.0 mL of 3.8 ×× 10−5 M NaCN, the minimum lethal concentration of sodium cyanide in blood serum

(c) 5.50 L of 13.3 M H2CO, the formaldehyde used to “fix” tissue samples

(d) 325 mL of 1.8 ×× 10−6 M FeSO4, the minimum concentration of iron sulfate detectable by taste in drinking water

(a) 37.0 mol H2SO4;


3.63 ×× 103 g H2SO4;

(b) 3.8 ×× 10−6 mol NaCN;

1.9 ×× 10−4 g NaCN;

(c) 73.2 mol H2CO;

2.20 kg H2CO;

(d) 5.9 ×× 10−7 mol FeSO4;

8.9 ×× 10−5 g FeSO4

Calculate the number of moles and the mass of the solute in each of the following solutions:

(a) 325 mL of 8.23 ×× 10−5 M KI, a source of iodine in the diet

(b) 75.0 mL of 2.2 ×× 10−5 M H2SO4, a sample of acid rain

(c) 0.2500 L of 0.1135 M K2CrO4, an analytical reagent used in iron assays

(d) 10.5 L of 3.716 M (NH4)2SO4, a liquid fertilizer

Consider this question: What is the molarity of KMnO4 in a solution of 0.0908 g of KMnO4 in 0.500 L of solution?

(a) Outline the steps necessary to answer the question.

(b) Answer the question.

(a) Determine the molar mass of KMnO4; determine the number of moles of KMnO4 in the solution; from the number of moles and the volume of solution, determine the molarity; (b) 1.15 ×× 10−3 M

Consider this question: What is the molarity of HCl if 35.23 mL of a solution of HCl contain 0.3366 g of HCl?

(a) Outline the steps necessary to answer the question.

(b) Answer the question.

Calculate the molarity of each of the following solutions:

(a) 0.195 g of cholesterol, C27H46O, in 0.100 L of serum, the average concentration of cholesterol in human serum

(b) 4.25 g of NH3 in 0.500 L of solution, the concentration of NH3 in household ammonia

(c) 1.49 kg of isopropyl alcohol, C3H7OH, in 2.50 L of solution, the concentration of isopropyl alcohol in rubbing alcohol

(d) 0.029 g of I2 in 0.100 L of solution, the solubility of I2 in water at 20 °C

(a) 5.04 ×× 10−3 M;


(b) 0.499 M;

(c) 9.92 M;

(d) 1.1 ×× 10−3 M

Calculate the molarity of each of the following solutions:

(a) 293 g HCl in 666 mL of solution, a concentrated HCl solution

(b) 2.026 g FeCl3 in 0.1250 L of a solution used as an unknown in general chemistry laboratories

(c) 0.001 mg Cd2+ in 0.100 L, the maximum permissible concentration of cadmium in drinking water

(d) 0.0079 g C7H5SNO3 in one ounce (29.6 mL), the concentration of saccharin in a diet soft drink.

There is about 1.0 g of calcium, as Ca2+, in 1.0 L of milk. What is the molarity of Ca2+ in milk?

0.025 M

What volume of a 1.00-M Fe(NO3)3 solution can be diluted to prepare 1.00 L of a solution with a concentration of 0.250 M?

If 0.1718 L of a 0.3556-M C3H7OH solution is diluted to a concentration of 0.1222 M, what is the volume of the resulting solution?

0.5000 L

If 4.12 L of a 0.850 M-H3PO4 solution is be diluted to a volume of 10.00 L, what is the concentration of the resulting solution?

What volume of a 0.33-M C12H22O11 solution can be diluted to prepare 25 mL of a solution with a concentration of 0.025 M?

1.9 mL

What is the concentration of the NaCl solution that results when 0.150 L of a 0.556-M solution is allowed to evaporate until the volume is reduced to 0.105 L?

What is the molarity of the diluted solution when each of the following solutions is diluted to the given final volume?

(a) 1.00 L of a 0.250-M solution of Fe(NO3)3 is diluted to a final volume of 2.00 L

(b) 0.5000 L of a 0.1222-M solution of C3H7OH is diluted to a final volume of 1.250 L

(c) 2.35 L of a 0.350-M solution of H3PO4 is diluted to a final volume of 4.00 L

(d) 22.50 mL of a 0.025-M solution of C12H22O11 is diluted to 100.0 mL

(a) 0.125 M;


(b) 0.04888 M;

(c) 0.206 M;

(e) 0.0056 M

What is the final concentration of the solution produced when 225.5 mL of a 0.09988-M solution of Na2CO3 is allowed to evaporate until the solution volume is reduced to 45.00 mL?

A 2.00-L bottle of a solution of concentrated HCl was purchased for the general chemistry laboratory. The solution contained 868.8 g of HCl. What is the molarity of the solution?

11.9 M

An experiment in a general chemistry laboratory calls for a 2.00-M solution of HCl. How many mL of 11.9 M HCl would be required to make 250 mL of 2.00 M HCl?

What volume of a 0.20-M K2SO4 solution contains 57 g of K2SO4?

1.6 L

The US Environmental Protection Agency (EPA) places limits on the quantities of toxic substances that may be discharged into the sewer system. Limits have been established for a variety of substances, including hexavalent chromium, which is limited to 0.50 mg/L. If an industry is discharging hexavalent chromium as potassium dichromate (K2Cr2O7), what is the maximum permissible molarity of that substance?

Key Equations

Glossary

aqueous solution
solution for which water is the solvent
concentrated
qualitative term for a solution containing solute at a relatively high concentration
concentration
quantitative measure of the relative amounts of solute and solvent present in a solution
dilute
qualitative term for a solution containing solute at a relatively low concentration
dilution
process of adding solvent to a solution in order to lower the concentration of solutes
dissolved
describes the process by which solute components are dispersed in a solvent
molarity (M)
unit of concentration, defined as the number of moles of solute dissolved in 1 liter of solution
solute
solution component present in a concentration less than that of the solvent
solvent
solution component present in a concentration that is higher relative to other components

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