Chapter 9: Chemical Quantities

 

Vocabulary

mole

representative particle

Avogadro's number

6.02 x 1023

atoms

molecules

formula units

molar mass

gram atomic mass (gam)

gram molecular mass (gmm)

gram formula mass (gfm)

mole - mass conversions

standard temperature and pressure

STP (0C, 1 atm)

molar volume of gasses

percent composition

empirical formulas

molecular formulas

 

 

 

Important Identities

1 mole particles = 6.02 x 1023 particles 1 mole gas = 22.4 L (at STP)

 

The term mole is used to describe a certain amount of matter. That amount is 6.02 x 1023 of the smallest particles that make up that form of matter. Thus, one mole of an element contains 6.02 x 1023 atoms; one mole of molecular compounds contains 6.02 x 1023 molecules; and one mole of an ionic compound contains 6.02 x 1023 formula units.

 

1. How many moles of zinc are in 4.22 x 1027 atoms of zinc?

2. How many molecules of methane are there in 2.50 moles of methane, C4H4?

3. How many sodium ions are there in 0.482 mol of sodium sulfate (Na2SO4)?

4. How many phosphorus atoms are there in 1.75 mole of calcium phosphate, Ca3(PO4)2?

 

Atomic, molecular, and formula masses are all relative numbers. That is, they contain no units. Saying that the atomic mass of magnesium is 24 means that its atoms are twice as heavy as those of some other element (carbon) with an atomic mass of 12. It is possible, however, to assign units to atomic, molecular, and formula masses. When the unit grams is used with an atomic mass, we define the gram atomic mass (gam) of an element. The gram atomic mass is the mass of one mole of an element. Similarly the gram molecular mass (gmm) is the mass of one mole of a molecular compound, and the gram formula mass (gfm) is the mass of one mole of an ionic compound. The term molar mass is a general term that refers to the gram atomic, molecular, and formula mass of any substance.

 

5. What is the molar mass of phosphoric acid, H3PO4?

6. What is the gram formula mass of aluminum hydroxide?

7. What is the gram molecular mass of sucrose, C12H22O11?

8. What is the molar mass of iron (III) sulfate?

 

Chemists often need to know about the quantitative relationships among the elements and compounds involved in a chemical reaction. The relationships involve masses and/or volumes. For example, you might want to know what volume of gas is produced when a certain mass of compound is heated. The concept of moles is the key ideas that makes it possible to deal easily and efficiently with all mass and volume relationships in chemical reactions.

 

9. What is the volume at STP of 2.66 mol of methane (CH4) gas?

10. During an electroplating process, 5.8625 gm of silver is deposited on a steel bar. How many moles of silver is this?

11. How many moles of ammonia gas (NH3) are there in 135 L of ammonia at STP?

12. A nurse has been asked to get 0.0465 mole of quinine (C20H24N2O2). What mass of quinine should he obtain?

13. A small bottle in the chemistry stockroom contains 43.25 gm of nickel (II) carbonate. How many moles of nickel (II) carbonate is this?

14. How many oxygen atoms are there in 66.7 L of carbon dioxide at STP?

15. What is the mass of 33 L of hydrogen gas at STP?

16. A chemist is studying one of the five oxides of nitrogen: N2O, NO, N2O3, NO2, and N2O5. She learns that 250 mL of the gas has a mass of 0.355 gm. What oxide is she working with?

17. Which has a greater mass: 92.3 L of carbon dioxide at STP or 70.1 L of chlorine gas at STP?

18. What is the density of N2O, a gas at STP?

 

It is sometimes useful to know the percent composition of a compound. Percent composition refers to the number of grams of each element in 100 gm of the compound. Knowing a compound's percent composition is the first step in determining the formula of that compound. Percent composition is determined by analyzing a sample of the compound to see what mass of each element is present in the compound. For each element

Percent of element present = x 100%

 

19. Find the percent composition of a compound containing tin and chlorine if 18.35 gm of the compound contains 5.74 gm of tin.

20. Determine the percent composition of ammonia from its formula, NH3?

21. Calculate the percent of each element present in potassium phosphate from its formula.

22. From the formula for calcium acetate, calculate the mass of carbon that can be obtained from 65.3 gm of the compound.

 

A compound never before seen has just been prepared in a chemistry laboratory. What is the formula for that compound? In order to answer that question we must first determine the percent composition of the compound. From this information you can determine the compounds empirical formula. The empirical formula of a compound shows the simplest whole-number ratio of elements present in the compound. The first step is to calculate the number of moles of each element present in the compound. Next, you must find the mole ratio of the elements in the compound by dividing the number of moles of each element by the smallest number of moles. The whole number ratio you obtain gives you the subscripts for the empirical formula of the compound. The molecular formula may be the same as the empirical formula or some whole-number multiple of the empirical formula. In order to determine the compound's molecular formula, you need to know both the empirical formula and the formula mass of the compound.

 

23. Determine the empirical formula of a compound that contains 69.5 gm oxygen and 30.5 gm nitrogen.

24. Determine the empirical formula of a certain copper sulfide ore if a 7.68 gm sample of the compound contains 6.13 gm copper.

25. The gram formula mass of a compound is 166.3 gm. The compound contains 47.1% potassium, 14.5% carbon, and 38.4% oxygen. What is the molecular formula for the compound?

26. How many kilograms of iron can be recovered from 639 kilograms of iron (III) oxide?