Sample Problems

CHEMISTRY PART I

Protons, Neutrons and Electron

1. Indicate the number of protons, neutrons and electrons in 35

17Cl.

2. Write an appropriate symbol for the species with 37 protons, 61 neutrons and 47 electrons.

3. Determine the number of protons, neutrons and electrons in an ion of sulfur the carries the charge -2. 35

16S-2

Atomic Weight

1. Use the weighted average atomic mass of 6.941 to determine the natural abundances of lithium-6 (6.01513) and lithium 7(7.01601).

Law of Conservation of Mass

1. A 0.382 g sample of magnesium reacts with 2.652 g nitrogen gas. The sole product is magnesium nitride. After the reaction, the mass of unreacted nitrogen is 2.505 g. What mass of magnesium nitirde was produced?

2. A 7.12 g sample of magnesium is heated with 1.80 g of bromine. All the bromine is used up, and 2.07 g of magnesium bromide is the only product. What mass of magnesium remains unreacted?

Law of Constant Composition

1. A 0.100 g sample of magnesium, when combined with oxygen yields 0.166 g of magnesium oxide. Determine the mass of magnesium contained in 0.500 g of magnesium oxide.

2. We wish to make exactly 2.00 g of magnesium oxide. What masses of magnesium and oxygen must we combine to do this?

Relationships Derived from Chemical Formula

1. How many grams of C are contained in 75 mL of halothane, C2HBrClF3? D=1.871 g/mL

2. How many mL of halothane must be measured out to contain 100.0 g Br?

Number of Mols and the Total Number of Atoms, and Mass in Grams

1. How many atoms of gold are present in 5.07 x 10-3 mol Au?

2. What is the mass of 2.35 x10 -24 atoms of Cu?

3. How many Pb atoms are present in a small piece of lead with a volume of 0.105 cm3? d=11.34 g/cm3

4. Rhenium 187 is a radioactive isotope that can be used to determine the age of meteorites. A 0.100 mg sample of Re contains 2.02 x10 17 atoms of 187Re. What is the percent abundance of rhenium 187 in the sample?

Relating Molar Mass, Avogadro’s Constant and Formula Units

1. Zinc oxide is used in sunscreen preparation. What is the total number of ions present in a 1.0 g sample of ZnO?

2. How many grams of MgCl2 should you measure out if you seek a sample with a total of 5.0 x 10-23

Cl- ions?

Mass Percent Composition of a Compound

1. Calculate the mass percent composition of acetic acid. HCH3O2.

Oxidation States

1. What is the oxidation state of the underlined element in each of the following:

1. S8

2. Cr2O72-

3. Cl2O4. KO2

5. S2O32-

6. KMnO4

Stoichiometric Calculation

1. How many grams of O2 are consumed per gram of octane C8H18 in the combustion of octane?

2. An alloy used in aircraft structures consists of 93.7 % Al and 6.3 % Cu by mass? The alloy has a density of 2.85 g/cm3. Assume all the Al but Cu reacts with HCl. What volume of the Al-Cu alloy must be dissolved in an excess of HCl to produce 1.00 g H2?

Calculating Molarity from Measured Units

1. A 22.3 g sample of acetone (CH3)2CO is dissolved in enough water to produce 1.25 L of solution. What is the molarity of acetone in this solution?

2. 15.0 mL of acetic acid (d=1.048 g/mL) is dissolved in enough water to produce 500.00 mL of solution. What is the molarity of acetic acid in the solution?

3. An aqueous solution saturated with NaNO3 at 25oC is 10.8 M NaNO3. How many grams of NaNO3 are present in 125 mL of this solution at 25oC?

4. How many grams of Na2SO4.H2O are needed to prepare 355 mL of 0.443M Na2SO4?

Solution Dilution

1. A 15.00 mL sample of 0.450 M K2CrO4 is diluted to 100.00 mL. What is the concentration of the new solution?

2. When left in an open beaker for a period of time, the volume of 275 mL of 0.105 M NaCl is found to decrease to 237 mL due to evaporation of water. What is the new concentration of the solution?

Molarity

1. How many mL of 0.250 M K2CrO4 must be added to excess AgNO3 to produce 1.50 g Ag2CrO4?

Limiting Reactant

1. If 215 g P4 is allowed to react with 725 Cl2, how many grams of PCl3 are formed?

2. If 2.2 g H2 and 154 g O2 are allowed to react, which gas and what mass of that gas remains after the reaction?

Redox Reactions

1. Balance the redox reaction in acidic medium:

SO32- + MnO4

- SO4

2- + Mn2+

2. Balance the redox reaction in basic medium

SO32- + MnO4

- SO4

2- + Mn2+