StoichiometryMole-Mass Examples

The solution procedure used below involves making two ratios and setting them equal to each other. This is called a proportion. One ratio will come from the coefficients of the balanced equation and the other will be constructed from the problem. The ratio that is set up using data in the problem will almost always be the one with an unknown in it.

You will then cross-multiply and divide to get the answer.

The above is the technique used in mole-mole problems.

In this tutorial, there will be an addition to the technique used in mole-mole problems. One of the values involved will be expressed in grams. This could happen two different ways:

(Given Grams, Calculate Moles) suppose you are given a mass in the problem statement. You will need to convert this to moles FIRST. You do this by dividing the mass given by the molar mass of the substances. This technique is covered in the mole section of the ChemTeam. Click this link to go to the proper mole file for review.

(Given Moles, Calculate Grams) suppose you are asked for a mass as an answer to the problem. You will will first use the ratio and proportion to calculate an answer in moles. Then, that answer in moles will be converted to grams and this will be the final answer. You do this by multiplying the moles by the molar mass of the substance. This technique is covered in the mole section of the ChemTeam. Click this link to go to the proper mole file for review.

Here is the equation we'll use for the first three examples:

2KClO3 ---> 2KCl + 3O2

Example #1: 1.50 mol of KClO3 decomposes. How many grams of O2 will be produced?

Solution:

1) Let's use this ratio to set up the proportion:

 KClO3 –––––– O2

2) That means the ratio from the equation is:

 2 mol KClO3 ––––––––––– 3 mol O2

3) The ratio from the data in the problem will be:

 1.50 mol KClO3 ––––––––––––– x

4) The proportion (setting the two ratios equal) is:

 1.50 mol KClO3 2 mol KClO3 –––––––––––– = ––––––––––– x 3 mol O2

Cross-multiplying and dividing gives x = 2.25 mol of O2 produced.

5) The last step is to convert from moles to grams:

(2.25 mol) (32.00 g/mol) = 72.0 g (to three sig figs)

32.00 g/mol is the molar mass of O2

Example #2: If 80.0 grams of O2 was produced, how many moles of KClO3 decomposed?

1) Let's use this ratio to set up the proportion:

 O2 –––––– KClO3

2) That means the ratio from the equation is:

 3 mol O2 ––––––––––– 2 mol KClO3

3) Before creating the molar ratio from information in the problem, we must convert the grams given in the problem to moles:

80.0 g ÷ 32.00 g/mol = 2.50 mol of O2

Remember that oxygen as an element is written O2.

4) The ratio from the data in the problem will be:

 2.50 mol O2 ––––––––– x

5) The proportion (setting the two ratios equal) is:

 2.50 mol O2 3 mol O2 –––––––––– = –––––––––– x 2 mol KClO3

Solving by cross-multiplying and dividing gives x = 1.67 mol of KClO3 decomposed.

Example #3: We want to produce 2.75 mol of KCl. How many grams of KClO3 would be required?

Solution:

1) Let's use this ratio to set up the proportion:

 KCl –––––– KClO3

2) The ratio from the equation is:

 2 mol KCl –––––––––– 2 mol KClO3

3) The ratio from the data in the problem will be:

 2.75 mol KCl ––––––––––– x

4) The proportion (setting the two ratios equal) is:

 2.75 mol KCl 2 mol KCl –––––––––––– = –––––––––– x 2 mol KClO3

Solving the above gives 2.75 mol of KClO3.

5) The question wants grams for the answer:

(2.75 mol) (122.55 g/mol) = 337 g

122.55 g/mol is the molar mass of KClO3.

Here's the equation to use for the next three examples:

2H2 + O2 ---> 2H2O

Example #4: How many grams of H2O are produced when 2.50 moles of oxygen are used?

Solution:

1) Here are the two substances in the molar ratio I used:

 O2 –––––– H2O

2) The ratio from the equation is:

 1 mol O2 ––––––––– 2 mol H2O

3) The molar ratio from the problem data is:

 2.50 mol O2 –––––––––– x

4) The proportion to use is:

 2.50 mol O2 1 mol O2 –––––––––– = –––––––– x 2 mol H2O

x = 5.00 mol H2O

5) Convert moles to grams:

(5.00 mol) (18.0 g/mol) = 90.0 g.

Example #5: If 3.00 moles of H2O are produced, how many grams of oxygen must be consumed?

Solution:

1) Here are the two substances in the molar ratio I used:

 O2 –––––– H2O

2) The ratio from the equation is:

 1 mol O2 ––––––––– 2 mol H2O

3) The molar ratio from the problem data is:

 x ––––––––––– 3.00 mol H2O

4 The proportion to use is:

 x 1 mol O2 ––––––––––– = –––––––– 3.00 mol H2O 2 mol H2O

x = 1.50 mol O2

5) Convert moles to grams:

(1.50 mol) (32.0 g/mol) = 48.0 g.

Example #6: How many grams of hydrogen gas must be used, given the data in example #5?

1) Here are the two substances in the molar ratio I used:

 H2 –––– O2

2) The ratio from the equation is:

 2 mol H2 ––––––– 1 mol O2

3) The molar ratio from the problem data is:

 x ––––––––– 1.50 mol O2

4) The proportion to use is:

 x 2 mol H2 ––––––––– = ––––––– 1.50 mol O2 1 mol O2

x = (3.00 mol) (2.016 g/mol) = 6.05 g (to three sig figs)

5) The H2 : H2O ratio of 2/2 could have been used also. In that case, the ratio from the problem would have been 3.00 over x, since you were now using the water data and not the oxygen data.

Example #7: How many molecules (not moles) of NH3 are produced from 1.75 x 10¯4 g of H2? Express your answer numerically as the number of molecules.

Solution:

1) Write the balanced chemical equation for the reaction:

3H2 + N2 ---> 2NH3

2) Determine moles of H2 present:

 1.75 x 10¯4 g ––––––––––– = 0.000086806 mol 2.016 g/mol

3) Use the 2:3 NH3 to H2 molar ratio to determine moles of NH3:

 2 mol NH3 x ––––––––– = ––––––––––––––––– 3 mol H2 0.000086806 mol H2

x = 0.0000578707 mol of NH3

4) Use Avogadro's Number to determine number of molecules:

(0.0000578707 mol) (6.022 x 1023 molecules/mol) = 3.48 x 1019 molecules

5) Set up using dimensional analysis:

 1 mol H2 2 mol NH3 6.022 x 1023 molecules NH3 1.75 x 10¯4 g H2 x ––––––– x ––––––– x –––––––––––––––––––––– = 3.48 x 1019 molecules of NH3 2.016 g H2 3 mol H2 mol NH3

Example #8: The balanced chemical equation for the reaction between sodium carbonate and hydrochloric acid is:

CaCO3(s) + 2HCl(aq) ---> CaCl2(s) + CO2(g) + H2O(ℓ)

If the equation is interpreted on the mole level, what is the total mass of products in the balanced equation?

Solution:

1) "interpreted on the mole level" means this:

one mole of CaCO3 reacts with two moles of HCl to produce one mole of CaCl2, one mole of CO2, and one mole of H2O

2) We want the combined weight of the reactants: one mole of CaCl2, one mole of CO2, and one mole of H2O:

CaCl2 ---> 110.984 g/mol
one mole of CO2 ---> 44.009 g/mol
one mole of H2O ---> 18.015 g/mol

The sum is 173.008 g

3) Using the Law of Conservation of Mass, the sum of the reactants (one mole of CaCO3 and two moles of HCl) will also add up to 173.008. You may check that for yourself, if you so desire.

Example #9: 1.5503 grams of magnesium reacts with hydrochloric acid according to this balanced reaction:

Mg(s) + 2HCl(aq) ---> MgCl2(aq) + H2(g)

How many moles of hydrogen gas will be produced?

Solution:

Comment: you will first convert grams of Mg to moles of Mg, then, using the Mg to H2 ratio of moles (found in the balanced chemical equation) you convert to moles of H2.

1) Grams to moles:

1.5503 g / 24.305 g/mol = 0.063785 mol of Mg

2) Use molar ratio:

The Mg:H2 molar ratio is 1:1

1 is to 1 as 0.063785 mol is to x

x = 0.063785 mol of H2

3) Many teachers (and textbooks) set up problems like the above problem using "dimensional analysis" (also called the factor-label method or the unit-factor method). Here's the set up, formatted two different ways:

1.5503 g Mg x (1 mol Mg / 24.305 g Mg) x (1 mol H2 / 1 mol Mg) = 0.063785 mol H2

 1 mol Mg 1 mol H2 1.5503 g x ––––––– x ––––––– = 0.063785 mol H2 24.305 g Mg 1 mol Mg

Example #10: Calculate the moles of NO produced by the reaction of 27.85 grams of N2 according to the following chemical equation:

N2 + O2 ---> 2NO

Comment: nothing is said about the oxygen. The usual assumption is there is sufficient oxygen for all the nitrogen to be used up. If you assume insufficient oxygen is present, then the problem cannot be solved, so you don't have to do it.

Have fun justifying the second assumption to your teacher!

Solution:

 1 mol N2 2 mol NO 27.85 g N2 x –––––––––– x ––––––– = 1.988 mol NO 28.014 g N2 1 mol N2

This is a link to an outside source that contains two mole-mass problems along with the solutions, as well as some introductory explanation.