1,001 Chemistry Practice Problems For Dummies (2014)

Part I. The Questions

Chapter 10. Molar Calculations

Moles are extremely important to many chemical calculations. Sometimes you need to find moles, and in other cases, moles are necessary to find something else. The molar mass is useful to convert mass to moles or to convert moles to mass. Converting moles to mass is useful in determining the percent composition and percent yield. Converting mass to moles is useful in determining empirical and molecular formulas and in finding the limiting reactant.

The Problems You’ll Work On

In this chapter, you work with molar calculations in the following ways:

check Calculating molar mass

check Finding percent composition

check Writing empirical and molecular formulas

check Completing molar calculations

check Finding the percent yield

check Working with limiting reactants

Note: For access to the periodic table, see the Appendix.

What to Watch Out For

Remember the following when working on molar calculations:

check Reducing the empirical formula isn’t possible.

check The molecular and empirical formulas may be the same.

check Moles may be the answer, but more often, moles are an intermediate. When in doubt, change to moles.

check A percent yield greater than 100 percent indicates that you (or the experimenter) made a mistake.

check The limiting reactant is the key for all subsequent calculations.

check The molar mass has units of grams/mole.

Calculating Molar Mass

622–636 Calculate the molar mass.

622. What is the molar mass of KCl rounded to two decimal places?

623. What is the molar mass of CaO rounded to two decimal places?

624. What is the molar mass of AlF3 rounded to two decimal places?

625. What is the molar mass of B2Orounded to two decimal places?

626. What is the molar mass of CBr4 rounded to two decimal places?

627. What is the molar mass of NH4Cl rounded to two decimal places?

628. What is the molar mass of Ba(NO3)2 rounded to two decimal places?

629. What is the molar mass of (NH4)2SO4 rounded to two decimal places?

630. What is the molar mass of silver sulfide rounded to two decimal places?

631. What is the molar mass of sodium carbonate rounded to two decimal places?

632. What is the molar mass of Al2(C2O4)3 rounded to two decimal places?

633. What is the molar mass of Zn(NH3)4Cl2 rounded to two decimal places?

634. What is the molar mass of CuSO4∙5H2O rounded to two decimal places?

635. What is the molar mass of manganese(II) nitrate hexahydrate rounded to two decimal places?

636. What is the molar mass of iron(II) phosphate octahydrate rounded to two decimal places?

Finding Mass Percent

637–651 Find the percent composition of compounds.

637. What is the mass percent of Na in NaBr rounded to two decimal places?

638. What is the mass percent of Sr in SrS rounded to two decimal places?

639. What is the mass percent of Cl in KClO3 rounded to two decimal places?

640. What is the mass percent of O in CaC2O4 rounded to two decimal places?

641. What is the mass percent of S in Na2S2O3 rounded to two decimal places?

642. What is the mass percent of N in NH4NO3 rounded to two decimal places?

643. What is the mass percent of Li in lithium hydrogen carbonate rounded to two decimal places?

644. What is the mass percent of silver in silver sulfide rounded to two decimal places?

645. What is the mass percent of each element in aluminum hydroxide rounded to two decimal places?

646. What is the mass percent of each element in zinc iodate rounded to two decimal places?

647. Rank the following compounds in order of increasing percent nitrogen content:

NO, NO2, N2O, N2O3, N2O5

648. Rank the following compounds in order of decreasing percent carbon content:

CH4, CCl4, C2H6, K2CO3, CaC2O4

649. Rank the following compounds in order of increasing percent sulfur content:

K2S, K2SO3, K2SO4, K2S2O3, KSCN

650. What is the mass percent of water in CuSO4·5H2O rounded to two decimal places?

651. What is the mass percent of oxygen in sodium phosphate dodecahydrate rounded to two decimal places? (Hint: The prefix dodeca- means 12.)

Empirical Formulas

652–656 Find the empirical formula.

652. Which of the following compounds could be correctly identified as an empirical formula?

C2H2, C2H4, C2H6, C2H3O2, C4H6O4

653. What is the empirical formula of a compound containing 85.62% C and 14.38% H?

654. What is the empirical formula of a compound containing 10.06 g of carbon, 0.84 g of hydrogen, and 89.09 g of chlorine?

655. A compound contains 1.4066 g of iron, 1.3096 g of chromium, and 1.6118 g of oxygen. What is the empirical formula of the compound?

656. A 100.00-g sample of a hydrate lost 18.73 g of water when heated. The anhydrous salt contains 43.82 g of cadmium and 12.50 g of sulfur, and the remaining mass is oxygen. What is the empirical formula of the anhydrous salt?

Molecular Formulas

657–661 Answer the questions about molecular formulas.

657. Which of the following pairs of compounds are correctly matched empirical and molecular formulas?

I. CH and CH4

II. CH and C2H2

III. C3H4 and C3H8

658. Which of the following pairs of compounds are correctly matched empirical and molecular formulas?

I. CO and CO2

II. CH2 and C6H12

III. CH2O and C6H12O6

659. If the empirical formula for a compound is CH and the molar mass of the compound is 78.11 g/mol, what is the molecular formula?

660. What is the molecular formula of a compound with a molar mass of 174.20 g/mol and with a mass percent composition of 41.37% carbon, 8.10% hydrogen, 18.39% oxygen, and 32.16% nitrogen?

661. A compound contains 23.25% carbon, 1.95% hydrogen, 17.20% oxygen, and 3.01% nitrogen, and the rest is iodine. If the compound has a molar mass of approximately 465 g/mol, what are the empirical and molecular formulas of the compound?

Mole Calculations

662–711 Complete the calculations involving the mole.

662. How many moles of NaCl are in 10.0 g of NaCl?

663. How many moles of BH3 are in 5.00 g of BH3?

664. How many moles of Na2CO3 are in 275 g of Na2CO3?

665. How many moles of NH4OH are in 400. g of NH4OH?

666. How many moles are in 99 g of KMnO4?

667. How many moles are in 150. g of beryllium sulfide?

668. How many moles are in 25.0 g of aluminum oxide?

669. How many moles are in 180. g of carbon tetrachloride?

670. How many moles are in 320. g of calcium phosphide?

671. How many moles are in 1.70 g of magnesium permanganate?

672. How many grams are in 5.25 mol of FeO?

673. How many grams are in 0.750 mol of S2Cl2?

674. How many grams are in 100. mol of KI?

675. How many grams are in 42.3 mol of CaCO3?

676. How many grams are in 7.9 mol of Al(NO3)3?

677. How many grams are in 0.15 mol of lead(II) nitrate?

678. How many grams are in 12 mol of tetraphosphorus decoxide?

679. How many grams are in 0.25 mol of copper(I) oxide?

680. How many kilograms are in 248 mol of zinc hydroxide?

681. How many kilograms are in 367 mol of ammonium sulfate?

682. How many atoms are in 0.250 g of He?

683. How many atoms are in 30. g of Al?

684. How many molecules are in 200. g of carbon dioxide?

685. How many molecules are in 0.050 g of ­dinitrogen tetroxide?

686. How many atoms are in 250. g of sulfur, S8?

687. How many grams are in 9.5 × 1026 molecules of Br2?

688. How many grams are in 8.306 × 1021 atoms of Ni?

689. How many grams are in 3.00 × 1023 molecules of C6H12O6?

690. What is the mass of 65,000,000 atoms of krypton gas?

691. What is the mass of 4.0 × 1022 formula units of AuF3, gold(III) fluoride?

692. Given the following equation, how many moles of ammonia, NH3, can be produced from 5.00 mol of nitrogen gas if there’s an excess of hydrogen gas?

9781118549322-eq10001.eps

693. Given the following balanced equation, how many moles of ammonia, NH3, can be produced from 12.0 mol of hydrogen gas if there’s an excess of nitrogen gas?

9781118549322-eq10002.eps

694. Given the following balanced equation, how many moles of nitrogen gas would be necessary to make 24.0 mol of ammonia, NH3, if there’s an excess of hydrogen gas?

9781118549322-eq10003.eps

695. Given the following balanced equation, how many moles of hydrogen gas would be necessary to make 36.0 mol of ammonia if there’s an excess of nitrogen gas?

9781118549322-eq10004.eps

696. If 3.00 mol of propane, C3H8, undergoes complete combustion according to the following reaction, how many moles of each product are produced?

9781118549322-eq10005.eps

697. According to the following reaction, what is the maximum number of grams of KCl that can be produced from 5.0 g of KClO3?

9781118549322-eq10006.eps

698. According to the following reaction, what is the maximum number of grams of O2 that can be produced from 5.0 g of KClO3?

9781118549322-eq10007.eps

699. According to the following reaction, how many grams of CaCO3 can be produced from 20.0 g of Ca(OH)2?

9781118549322-eq10008.eps

700. According to the following reaction, how many grams of NaCl can be produced from 80.0 g of NaOH?

9781118549322-eq10009.eps

701. Potassium phosphate plus silver nitrate forms potassium nitrate and silver phosphate. How many grams of silver phosphate can be formed from 5.6 g of silver nitrate and an excess of potassium phosphate?

702. Sodium hydroxide plus copper(II) sulfate forms copper(II) hydroxide and sodium sulfate. With an excess of copper(II) sulfate, how many grams of sodium hydroxide are needed to make 10.0 g of copper(II) hydroxide?

703. When the following reaction is completed and balanced, how many grams of zinc are needed to react with 74.5 g of hydrogen chloride?

9781118549322-eq10010.eps

704. When the following reaction is completed and balanced, how many grams of potassium nitrate will be made when 220 g of calcium phosphate is produced?

9781118549322-eq10011.eps

705. When the following reaction is completed and balanced, what is the minimum number of grams of each reactant needed to make 0.25 g of barium dichromate?

barium chloride + potassium dichromate 9781118549322-eq10012.eps

706. When the following reaction is completed and balanced, what is the maximum number of grams of product that can be made if there are 50.0 g of aluminum and 150. g of bromine? (Hint: The reaction has only one product.)

9781118549322-eq10013.eps

707. When the following reaction is balanced, how many molecules of O2 do you need to produce 8.02 × 1024 molecules of P2O5?

9781118549322-eq10014.eps

708. According to the following reaction, how many molecules of H2O can be produced from 1.69 × 1022 molecules of HNO3?

9781118549322-eq10015.eps

709. Barium sulfite decomposes to form barium oxide and sulfur dioxide gas. How many molecules of barium sulfite are needed to make 3.16 × 1021 molecules of sulfur dioxide?

710. Iodine gas plus chlorine gas makes iodine chloride gas. How many molecules of iodine are needed to make 5.28 × 1024 molecules of iodine chloride?

711. According to the following reaction, what is the minimum number of molecules of octane, C8H18, needed to make 4.52 × 1024 molecules of each product, assuming complete combustion of the octane?

9781118549322-eq10016.eps

Percent Yield

712–716 Complete the calculations related to percent yield.

712. If the theoretical yield of a product in a reaction was 1.358 g and the actual yield is 1.146 g, what is the percent yield of the product?

713. A student produced 5.15 g of a product during a chemistry experiment. The theoretical yield was 4.95 g. What is the percent yield of the product?

714. If 316 g of ammonia produces 1,225 g of ammonium bromide by reacting with excess hydrogen bromide, what is the percent yield of this reaction?

9781118549322-eq10017.eps

715. Ammonium bromide can be produced according to the following reaction:

9781118549322-eq10018.eps

What is the percent yield of this reaction if 623 g of ammonia produces 2,341 g of ammonium bromide?

716. A student performed a precipitation reaction using 0.527 g of BaCl2 dissolved in water with an excess of Na2SO4. If 0.551 g of BaSO4 is recovered, what was the percent yield of BaSO4?

Limiting Reactants

717–721 Perform calculations with limiting reactants.

717. According to the following balanced reaction, what is the maximum number of grams of SOthat can be prepared from 45.0 mol of SO2 and 25.0 mol of O2?

9781118549322-eq10019.eps

718. According to the following unbalanced reaction, what is the maximum amount of NaCl that can be prepared from 106 g of Cl2 and 154 g of NaClO2?

9781118549322-eq10020.eps

719. Aluminum combines with oxygen gas to form aluminum oxide, Al2O3. If 100.0 g of aluminum reacts with 100.0 g of oxygen, how many grams of aluminum oxide are produced, and which reactant is the limiting reactant?

720. A solution containing 155 g of KI is added to a solution containing 175 g of nitric acid and reacts according to the following equation:

9781118549322-eq10021.eps

How many grams of NO are produced, and which reactant is in excess?

721. A mixture containing 50.0 g of hydrogen gas and 50.0 g of oxygen gas is sparked and allowed to react. Which reactant is the limiting reactant, how much water is produced, and how much of the excess reactant remains?