{"id":633,"date":"2016-01-11T20:00:48","date_gmt":"2016-01-11T20:00:48","guid":{"rendered":"https:\/\/opentextbc.ca\/introductorychemistryclone\/chapter\/concentrations-as-conversion-factors-2\/"},"modified":"2020-04-20T16:36:41","modified_gmt":"2020-04-20T16:36:41","slug":"concentrations-as-conversion-factors","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/introductorychemistryclone\/chapter\/concentrations-as-conversion-factors\/","title":{"raw":"Concentrations as Conversion Factors","rendered":"Concentrations as Conversion Factors"},"content":{"raw":"
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Learning Objectives<\/h3>\r\n1. Apply concentration units as conversion factors.\r\n\r\n<\/div>\r\n<\/div>\r\n

Concentration can be a conversion factor between the amount of solute and the amount of solution or solvent (depending on the definition of the concentration unit). As such, concentrations can be useful in a variety of stoichiometry problems. In many cases, it is best to use the original definition of the concentration unit; it is that definition that provides the conversion factor.<\/p>\r\n

A simple example of using a concentration unit as a conversion factor is one in which we use the definition of the concentration unit and rearrange; we can do the calculation again as a unit conversion, rather than as a definition. For example, suppose we ask how many moles of solute are present in 0.108 L of a 0.887 M NaCl solution. Because 0.887 M means 0.887 mol\/L, we can use this second expression for the concentration as a conversion factor:<\/p>\r\n0.108\u00a0L\u00a0NaCl \u00d7 (0.887\u00a0mol\u00a0NaCl \/(1 L\u00a0NaCl)) = 0.0958\u00a0mol\u00a0NaCl<\/span>\r\n

(There is an understood 1 in the denominator of the conversion factor.) If we used the definition approach, we get the same answer, but now we are using conversion factor skills. Like any other conversion factor that relates two different types of units, the reciprocal of the concentration can be also used as a conversion factor.<\/p>\r\n\r\n

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Example 10<\/h3>\r\n

Using concentration as a conversion factor, how many liters of 2.35 M CuSO4<\/sub> are needed to obtain 4.88 mol of CuSO4<\/sub>?<\/p>\r\n

Solution<\/p>\r\n

This is a one-step conversion, but the concentration must be written as the reciprocal for the units to work out:<\/p>\r\n4.88\u00a0mol\u00a0CuSO4<\/sub>\u00d7(1\u00a0L \/ 2.35\u00a0mol) = 2.08\u00a0L\u00a0of\u00a0solution<\/span>\r\n

Test Yourself<\/em><\/p>\r\n

Using concentration as a conversion factor, how many liters of 0.0444 M CH2<\/sub>O are needed to obtain 0.0773 mol of CH2<\/sub>O?<\/p>\r\n

Answer<\/em><\/p>\r\n

1.74 L<\/p>\r\n\r\n<\/div>\r\n

Of course, once quantities in moles are available, another conversion can give the mass of the substance, using molar mass as a conversion factor.<\/p>\r\n\r\n

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Example 11<\/h3>\r\n

What mass of solute is present in 0.765 L of 1.93 M NaOH?<\/p>\r\n

Solution<\/p>\r\n

This is a two-step conversion, first using concentration as a conversion factor to determine the number of moles and then the molar mass of NaOH (40.0 g\/mol) to convert to mass:<\/p>\r\n0.765\u00a0L \u00d7 (1.93\u00a0mol\u00a0NaOH \/ L\u00a0solution) \u00d7 (40.0\u00a0g\u00a0NaOH \/ 1\u00a0mol\u00a0NaOH) = 59.1\u00a0g\u00a0NaOH<\/span>\r\n

Test Yourself<\/em><\/p>\r\n

What mass of solute is present in 1.08 L of 0.0578 M H2<\/sub>SO4<\/sub>?<\/p>\r\n

Answer<\/em><\/p>\r\n

6.12 g<\/p>\r\n\r\n<\/div>\r\n

More complex stoichiometry problems using balanced chemical reactions can also use concentrations as conversion factors. For example, suppose the following equation represents a chemical reaction:<\/p>\r\n2 AgNO3<\/sub>(aq) +\u00a0CaCl2<\/sub>(aq) \u2192\u00a02 AgCl(s) +\u00a0Ca(NO3<\/sub>)2<\/sub>(aq)<\/span><\/span>\r\n

If we wanted to know what volume of 0.555 M CaCl2<\/sub> would react with 1.25 mol of AgNO3<\/sub>, we first use the balanced chemical equation to determine the number of moles of CaCl2<\/sub> that would react and then use concentration to convert to liters of solution:<\/p>\r\n1.25\u00a0mol\u00a0AgNO3\u00a0<\/sub>\u00d7 (1\u00a0mol\u00a0CaCl2\u00a0<\/sub>\/ 2\u00a0mol\u00a0AgNO3<\/sub>) \u00d7 (1\u00a0L\u00a0solution \/ 0.555\u00a0mol\u00a0CaCl2<\/sub>) =1.13\u00a0L\u00a0CaCl2<\/sub><\/span>\r\n

This can be extended by starting with the mass of one reactant, instead of moles of a reactant.<\/p>\r\n\r\n

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Example 12<\/h3>\r\n

What volume of 0.0995 M Al(NO3<\/sub>)3<\/sub> will react with 3.66 g of Ag according to the following chemical equation?<\/p>\r\n3 Ag(s) +\u00a0Al(NO3<\/sub>)3<\/sub>(aq) \u2192\u00a03 AgNO3<\/sub> +\u00a0Al(s)<\/span><\/span>\r\n

Solution<\/p>\r\n

Here, we first must convert the mass of Ag to moles before using the balanced chemical equation and then the definition of molarity as a conversion factor:<\/p>\r\n3.66\u00a0g\u00a0Ag \u00d7 (1\u00a0\u00a0mol\u00a0Ag \/ 107.97\u00a0g\u00a0Ag) \u00d7 (1\u00a0mol\u00a0Al(NO3<\/sub>)3 <\/sub>\/\u00a0<\/sub>3\u00a0mol\u00a0Ag) \u00d7 (1\u00a0L\u00a0solution \/ 0.0995\u00a0mol\u00a0Al(NO3<\/sub>)3<\/sub>) = 0.114\u00a0L<\/span>\r\n

Test Yourself<\/em><\/p>\r\n

What volume of 0.512 M NaOH will react with 17.9 g of H2<\/sub>C2<\/sub>O4<\/sub>(s) according to the following chemical equation?<\/p>\r\nH2<\/sub>C2<\/sub>O4<\/sub>(s) +\u00a02 NaOH(aq) \u2192\u00a0Na2<\/sub>C2<\/sub>O4<\/sub>(aq) +\u00a02 H2<\/sub>O(\u2113)<\/span><\/span>\r\n

Answer<\/em><\/p>\r\n

0.777 L<\/p>\r\n\r\n<\/div>\r\n

We can extend our skills even further by recognizing that we can relate quantities of one solution to quantities of another solution. Knowing the volume and concentration of a solution containing one reactant, we can determine how much of another solution of another reactant will be needed using the balanced chemical equation.<\/p>\r\n\r\n

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Example 13<\/h3>\r\n

A student takes a precisely measured sample, called an aliquot<\/em>, of 10.00 mL of a solution of FeCl3<\/sub>. The student carefully adds 0.1074 M Na2<\/sub>C2<\/sub>O4<\/sub> until all the Fe3+<\/sup>(aq) has precipitated as Fe2<\/sub>(C2<\/sub>O4<\/sub>)3<\/sub>(s). Using a precisely measured tube called a burette, the student finds that 9.04 mL of the Na2<\/sub>C2<\/sub>O4<\/sub> solution was added to completely precipitate the Fe3+<\/sup>(aq). What was the concentration of the FeCl3<\/sub> in the original solution? (A precisely measured experiment like this, which is meant to determine the amount of a substance in a sample, is called a titration<\/em>.) The balanced chemical equation is as follows:<\/p>\r\n2 FeCl3<\/sub>(aq) +\u00a03 Na2<\/sub>C2<\/sub>O4<\/sub>(aq) \u2192\u00a0Fe2<\/sub>(C2<\/sub>O4<\/sub>)3<\/sub>(s) +\u00a06 NaCl(aq)<\/span><\/span>\r\n

Solution<\/p>\r\n

First we need to determine the number of moles of Na2<\/sub>C2<\/sub>O4<\/sub> that reacted. We will convert the volume to liters and then use the concentration of the solution as a conversion factor:<\/p>\r\n9.04\u00a0mL\u00a0\u00d7 (1\u00a0L \/ 1,000\u00a0mL) \u00d7 (0.1074\u00a0mol\u00a0Na2<\/sub>C2<\/sub>O4\u00a0<\/sub>\/\u00a0<\/sub>L) = 0.000971\u00a0mol\u00a0Na2<\/sub>C2<\/sub>O4<\/sub><\/span>\r\n

Now we will use the balanced chemical equation to determine the number of moles of Fe3+<\/sup>(aq) that were present in the initial aliquot:<\/p>\r\n0.000971\u00a0mol\u00a0Na2<\/sub>C2<\/sub>O4<\/sub> \u00d7 (2\u00a0mol\u00a0FeCl3\u00a0<\/sub>\/ 3\u00a0mol\u00a0Na2<\/sub>C2<\/sub>O4<\/sub>) = 0.000647\u00a0mol\u00a0FeCl3<\/sub><\/span>\r\n

Then we determine the concentration of FeCl3<\/sub> in the original solution. Converting 10.00 mL into liters (0.01000 L), we use the definition of molarity directly:<\/p>\r\nM = mol \/ L = 0.000647\u00a0mol\u00a0FeCl3\u00a0<\/sub>\/ 0.01000\u00a0L = 0.0647\u00a0M\u00a0FeCl3<\/sub><\/span>\r\n

Test Yourself<\/em><\/p>\r\n

A student titrates 25.00 mL of H3<\/sub>PO4<\/sub> with 0.0987 M KOH. She uses 54.06 mL to complete the chemical reaction. What is the concentration of H3<\/sub>PO4<\/sub>?<\/p>\r\nH3<\/sub>PO4<\/sub>(aq) +\u00a03 KOH(aq) \u2192\u00a0K3<\/sub>PO4<\/sub>(aq) +\u00a03 H2<\/sub>O<\/span><\/span>\r\n

Answer<\/em><\/p>\r\n

0.0711 M<\/p>\r\n\r\n

\r\n\r\n[caption id=\"attachment_3251\" align=\"alignnone\" width=\"600\"]\"When<\/a> When a student performs a titration, a measured amount of one solution is added to another reactant.
\u201cChemistry titration lab\u201d by Kentucky Country Day is licensed under the Creative Commons Attribution-NonCommercial 2.0 Generic.[\/caption]\r\n\r\n<\/div>\r\n<\/div>\r\n

We have used molarity exclusively as the concentration of interest, but that will not always be the case. The next example shows a different concentration unit being used.<\/p>\r\n\r\n

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Example 14<\/h3>\r\n

H2<\/sub>O2<\/sub> is used to determine the amount of Mn according to this balanced chemical equation:<\/p>\r\n2 MnO4<\/sub>\u2212<\/sup>(aq) +\u00a05 H2<\/sub>O2<\/sub>(aq) +\u00a06H+<\/sup>(aq) \u2192\u00a02 Mn2+<\/sup>(aq) +\u00a05 O2<\/sub>(g) +\u00a08 H2<\/sub>O(\u2113)<\/span><\/span>\r\n

What mass of 3.00% m\/m H2<\/sub>O2<\/sub> solution is needed to react with 0.355 mol of MnO4<\/sub>\u2212<\/sup>(aq)?<\/p>\r\n

Solution<\/p>\r\n

Because we are given an initial amount in moles, all we need to do is use the balanced chemical equation to determine the number of moles of H2<\/sub>O2<\/sub> and then convert to find the mass of H2<\/sub>O2<\/sub>. Knowing that the H2<\/sub>O2<\/sub> solution is 3.00% by mass, we can determine the mass of solution needed:<\/p>\r\n0.355\u00a0mol\u00a0MnO4<\/sub>\u2212\u00a0<\/sup>\u00d7 (5\u00a0mol\u00a0H2<\/sub>O2 <\/sub>\/\u00a0<\/sub>2\u00a0mol\u00a0MnO4<\/sub>\u2212<\/sup>) \u00d7 (34.02\u00a0g\u00a0H2<\/sub>O2<\/sub>\u00a0\/\u00a0mol\u00a0H2<\/sub>O2<\/sub>) \u00d7 (100\u00a0g\u00a0solution \/ 3\u00a0g\u00a0H2<\/sub>O2<\/sub>) = 1,006\u00a0g\u00a0solution<\/span>\r\n

The first conversion factor comes from the balanced chemical equation, the second conversion factor is the molar mass of H2<\/sub>O2<\/sub>, and the third conversion factor comes from the definition of percentage concentration by mass.<\/p>\r\n

Test Yourself<\/em><\/p>\r\n

Use the balanced chemical reaction for MnO4<\/sub>\u2212<\/sup> and H2<\/sub>O2<\/sub> to determine what mass of O2<\/sub> is produced if 258 g of 3.00% m\/m H2<\/sub>O2<\/sub> is reacted with MnO4<\/sub>\u2212<\/sup>.<\/p>\r\n

Answer<\/em><\/p>\r\n

7.28 g<\/p>\r\n\r\n<\/div>\r\n

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Key Takeaways<\/h3>\r\n