| Question 1: A 1990 US penny (zinc plated with a thin layer of copper) with a mass of 2.552 g contains 2.488 g of zinc. What is the mass percent of (a) copper and (b) zinc? |
Answer to Question 1:
Mass of the penny = 2.552 g. (4 sig figs)
Mass of zinc in the penny = 2.488 g. (4 sig figs)
(a) What is the mass percent of copper?
This is a percent composition question.
The difference of the two given mass is the mass of the copper = 2.552 – 2.488 = 0.0640 g. (3 sig figs)
Mass percent of copper = (mass of copper / mass of penny) · 100
Mass percent of copper = 0.0640 g / 2.552 g · 100 = 2.51 % copper (3 sig figs)
(b) What is the mass percent of zinc?
This is a percent composition question.
Mass percent of zinc = (mass of zinc / mass of penny) · 100
Mass percent of zinc = 2.488 g / 2.552 g · 100 = 97.49 % zinc (4 sig figs)
Check:
When you add the mass percent of copper and mass percent of zinc, it should equal 100%.
2.51 % + 97.49 % = 100.00 %. — This is CORRECT!
Review Unit 5 section 5.8 (http://nobel.scas.bcit.ca/courses/wpmu/chem0011/unit-5/)
| Question 2: What is the mass percent of carbon in halothane, C2HBrClF3? The formula mass of halothane is 197.38 amu. |
Answer to Question 2:
This is a percent composition question.
Since the formula mass of halothane is 197.38 amu/formula unit, the molar mass of halothane is 197.38 g/mole.
The mass percent of carbon in halothane =
(2 mole of carbon atom/1 mole halothane)·(12.011 g/mole ) / 197.38 g/mole ) · 100 = 12.170 %
Although the question did not ask the mass percent of the other elements, we could calculate all of them by using the correct number of each element found in halothane and their respective atomic masses in the calculation.
The mass percent of hydrogen in halothane =
(1 mole of hydrogen atom/1 mole halothane)·(1.008 g/mole ) / 197.38 g/mole ) · 100 = 0.5107 %
The mass percent of bromine in halothane =
(1 mole of bromine atom/1 mole halothane)·(79.904 g/mole ) / 197.38 g/mole ) · 100 = 40.482 %
The mass percent of chlorine in halothane =
(1 mole of chlorine atom/1 mole halothane)·(35.453 g/mole ) / 197.38 g/mole ) · 100 = 17.962 %
The mass percent of fluorine in halothane =
(3 mole of fluorine atom/1 mole halothane)·(18.9984 g/mole ) / 197.38 g/mole ) · 100 = 28.8759 %
The sum of all the mass percentages =
12.170 % C + 0.5107 % H + 40.482 % Br + 17.962 % Cl + 28.8759 % F = 100.000 %
Review Unit 5 section 5.8 (http://nobel.scas.bcit.ca/courses/wpmu/chem0011/unit-5/)
| Question 3: A 1.912 g sample of calcium chloride is decomposed into its constitutional elements of calcium and chlorine gas. The solid is found to contain 0.690 g of calcium. a) Write a balanced reaction for this reaction. b) Calculate the mass percent composition of calcium and chlorine in the original sample. |
Answer to Question 3:
(a) The chemical reaction for the decomposition of calcium chloride, CaCl2 (s), produces calcium metal, Ca (s), and chlorine gas, Cl2 (g).
CaCl2 
Ca (s) + Cl2 (g)
Is the reaction balanced?
| Left-Hand-Side | Right-Hand-Side | |
|---|---|---|
| Ca | ||
| Cl |
Yes, the reaction is balanced!
(b) This is a percent composition question.
The molar mass of CaCl2 is 110.968 g/mole.
The mass percent of calcium in calcium chloride =
(1 mole of calcium atom/1 mole calcium chloride )·(40.08 g/mole ) / 110.986 g/mole ) · 100 = 36.11 %
The mass percent of chlorine in calcium chloride =
(2 mole of chlorine atom/1 mole calcium chloride )·(35.453 g/mole ) / 110.986 g/mole ) · 100 = 63.887 %
The mass of calcium in the original sample is (36.11 %)(1.912 g) = 0.6904 g.
The mass of chlorine in the original sample is (63.887 %)(1.912 g) = 1.222 g.
Review Unit 5 section 5.8 (http://nobel.scas.bcit.ca/courses/wpmu/chem0011/unit-5/)
Review Unit 6 section 6.1, 6.2, 6.3 (http://nobel.scas.bcit.ca/courses/wpmu/chem0011/unit-6/)
