1,001 Chemistry Practice Problems For Dummies (2014)

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:

Calculating molar mass

Finding percent composition

Writing empirical and molecular formulas

Completing molar calculations

Finding the percent yield

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:

Reducing the empirical formula isn’t possible.

The molecular and empirical formulas may be the same.

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

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

The limiting reactant is the key for all subsequent calculations.

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 B₂O₃ rounded 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?

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?

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?

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?

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

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?

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?

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

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

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?

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?

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

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.)

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

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

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?

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?

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

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 SO₃ that can be prepared from 45.0 mol of SO₂ and 25.0 mol of O₂?

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?

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:

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?