# Average Atomic Mass Problems

Eng Chem

Homework #1

**Due date: 22/03/2011**

**Average Atomic Mass Problems**

1. What is average atomic mass of Lithium if 7.42% exists as 6Li (6.015 g/mol) and 92.58% exists as 7Li (7.016 g/mol)?

2. Magnesium has three naturally occuring isotopes. 78.70% of Magnesium atoms exist as Magnesium-24 (23.9850 g/mol), 10.03% exist as Magnesium-25 (24.9858 g/mol) and 11.17% exist as Magnesium-26 (25.9826 g/mol). What is the average atomic mass of Magnesium?

3. Neon has two major isotopes, Neon-20 and Neon-22. Out of every 250 neon atoms, 225 will be Neon-20 (19.992 g/mol), and 25 will be Neon-22 (21.991 g/mol). What is the average atomic mass of Neon?

4. What is the atomic mass of Hafnium if out of every 200 atoms, 10 have mass 176.00 g/mol, 38 have mass 177.00 g/mol, 54 have mass 178.00 g/mol, 28 have mass 179.00 g/mol, and 70 have mass 180.00 g/mol?

5. In a sample of 200 Chlorine atoms, it is found that 151 are 35Cl (34.969 g/mol), and 49 are 37Cl (36.966 g/mol). What is the average atomic mass of Chlorine?

6. Without doing any math, are there more Bromine-79 atoms or more Bromine-80 atoms on earth? (Hint: look at the periodic table.)

**Stoichiometry Problems**

1a) How many moles of chlorine gas (Cl2) would react with 5 moles of sodium (Na) according

to the following chemical equation? (Balance equation.)

Na + Cl2 --> NaCl

1b) Using the equation (after it is balanced) above, determine the amount of product that can be

produced from 24.7 g Na.

1c) How many molecules of product would be produced from 24.7g Na?

______

2a) In the reaction 2C8H18 + 25O2 --> 16CO2 + 18 H2O, the ratio of volumes of O2 to CO2

is ______.

2b) If 27.3g of C8H18 are combusted, what mass of water will be produced?

2c) How many molecules of CO2 will be produced?

2d) How many atoms of H are in 2 mol of C8H18?

2e) What is the percentage, by mass, of the H in 2 mol of C8H18?

**Limiting Reagent Worksheet**

*All of the questions on this worksheet involve the following reaction: When copper (II) chloride reacts with sodium nitrate, copper (II) nitrate and sodium chloride are formed.*

1)Write the balanced equation for the reaction given above:

2)If 15 grams of copper (II) chloride react with 20 grams of sodium nitrate, how much sodium chloride can be formed?

3)What is the limiting reagent for the reaction in problem #2? ______

4)How much of the nonlimiting reagent is left over in this reaction?

5)If 11.3 grams of sodium chloride are formed in the reaction described in problem #2, what is the percent yield of this reaction?

**Molarity Practice Problems**

1)How many grams of potassium carbonate are needed to make 200 mL of a 2.5 M solution?

2)How many liters of 4 M solution can be made using 100 grams of lithium bromide?

3)What is the concentration of a 450 mL solution that contains 200 grams of iron (II) chloride?

4)How many grams of ammonium sulfate are needed to make a 0.25 M solution at a concentration of 6 M?

5)What is the concentration of a solution that has a volume of 2.5 L and contains 660 grams of calcium phosphate?

6)How many grams of copper (II) fluoride are needed to make 6.7 liters of a 1.2 M solution?

7)How many liters of 0.88 M solution can be made with 25.5 grams of lithium fluoride?

8)What is the concentration of a solution that with a volume of 660 that contains 33.4 grams of aluminum acetate?

9)How many liters of 0.75 M solution can be made using 75 grams of lead (II) oxide?

10)How many grams of manganese (IV) oxide are needed to make a 5.6 liters of a 2.1 M solution?

11)What is the concentration of a solution with a volume of 9 mL that contains 2 grams of iron (III) hydroxide?

12)How many liters of 3.4 M solution can be made using 78 grams of isopropanol (C3H8O)?

13) What is the concentration of a solution with a volume of 3.3 mL that contains 12 grams of ammonium sulfite?

**Dilutions Worksheet**

1)If I have 340 mL of a 0.5 M NaBr solution, what will the concentration be if I add 560 mL more water to it?

2)If I dilute 250 mL of 0.10 M lithium acetate solution to a volume of 750 mL, what will the concentration of this solution be?

3)If I leave 750 mL of 0.50 M sodium chloride solution uncovered on a windowsill and 150 mL of the solvent evaporates, what will the new concentration of the sodium chloride solution be?

4)To what volume would I need to add water to the evaporated solution in problem 3 to get a solution with a concentration of 0.25 M?