MU #1 The percent composition by mass of each element in a compound can be calculated mathematically.

See table T for Percent Composition formula

% composition by mass = (mass of part / mass of whole) x 100

Example

What is the percent composition by mass of H in H₂O?

Step 1: Calculate the mass of the element in the compound.

mass of H = 2 (1 g) = 2 g

Step 2: Calculate the formula mass of the compound.

mass of H₂O = 2 (1 g) + 16 g = 18 g

Step 3: Apply the formula

(2 g / 18g) x 100  = 11 %

Therefore, 11 % of the mass of the compound is made up of hydrogen.

Example

What is the percent by mass of water in hydrated gypsum, CaSO₄.2H₂O? Notice that there are 2 moles of H₂O for every mole of CaSO₄

The mass of 2 H₂O is 4(1g) + 2(16g) = 36 g

The molecular mass of CaSO₄.2H₂O is 40g + 32g + 4(16g) + 4(1g) + 2(16g) = 172 g

The % by mass of water  in CaSO₄.2H₂O is (36g / 172g) 100 = 21%

Example

How many grams of O can be produced from the decomposition of 50.g of H₂O?

Calculate the % composition of O in H₂O and multiply by the mass of the sample.

89% x 50.g =  45g of O

Problems

1. Find the % composition of each element in the following compounds; Mg(NO₃)₂, (NH₄)₃PO₄, and  Al₂(SO₄)₃ 

2. How much iron can be recovered from 25g of Fe₂O₃?

3. How much silver can be produced from 125g of Ag₂S?

TEST YOUR UNDERSTANDING

1/07

54 A hydrated compound contains water molecules within its crystal structure. The percent composition by mass of water in the hydrated compound CaSO₄.2H₂O has an accepted value of 20.9%. A student did an experiment and determined that the percent composition by mass of water in CaSO₄.2H₂O was 21.4%.

Calculate the percent error of the student’s experimental result. Your response must include both a correct numerical setup and the calculated result.

6/07

55 Determine the percent composition by mass of oxygen in the compound C₆H₁₂O₆.

8/07

Base your answers to questions 73 through 75 on the information below.

A hydrate is a compound that has water molecules within its crystal structure. The formula for the hydrate CuSO₄•5H₂O(s) shows that there are five moles of water for every one mole of CuSO₄(s). When CuSO₄•5H₂O(s) is heated, the water within the crystals is released, as represented by the balanced equation below.

CuSO₄•5H₂O(s) --> CuSO₄(s) + 5H₂O(g)

A student first masses an empty crucible (a heat-resistant container). The student then masses the crucible containing a sample of CuSO₄•5H₂O(s). The student repeatedly heats and masses the crucible and its contents until the mass is constant. The student’s recorded experimental data and calculations are shown below.

73 Identify the total number of significant figures recorded in the calculated mass of CuSO₄•5H₂O(s).

74 Use the student’s data to show a correct numerical setup for calculating the percent composition by mass of water in the hydrate.

75 Explain why the sample in the crucible must be heated until the constant mass is reached.

6/02

The percent by mass of hydrogen in NH₃ is equal to (1) 17/1 x 100    (2) 17/3 x 100        (3) 1/17 x 100     (4) 3/17 x 100

1/03

1.  A hydrate is a compound that includes water molecules within its crystal structure. During an experiment to determine the percent by mass of water in a hydrated crystal, a student found the mass of the hydrated crystal to be 4.10 grams. After heating to constant mass, the mass was 3.70 grams. What is the percent by mass of water in this crystal?  (1) 90.%     (2) 11%               (3) 9.8% (4) 0.40%

2.  What is the percent by mass of oxygen in H2SO₄ ? [formula mass = 98g]  (1) 16%            (2) 33%        (3) 65%                    (4) 98%

6/03

10. The percent by mass of calcium in the compound calcium sulfate (CaSO₄) is approximately  (1) 15% (2) 29%  (3) 34% (4) 47%

8/03

8 In which compound is the percent by mass of oxygen greatest? (1) BeO (2) MgO (3) CaO (4) SrO

1/04

Base your answers to questions 75 and 76 on the information below.
Gypsum is a mineral that is used in the construction industry to make drywall (sheetrock). The chemical formula for this hydrated compound is CaSO₄.2H₂O. A hydrated compound contains water molecules within its crystalline structure. Gypsum contains 2 moles of water for each 1 mole of calcium sulfate.

75 What is the gram formula mass of CaSO₄.2H₂O? ___________ g/mol
76

a Show a correct numerical setup for calculating the percent composition by mass of water in this compound.
b Record your answer. __________%

6/04

8 What is the percent by mass of oxygen in propanal, CH₃CH₂CHO? (1) 10.0% (3) 38.1% (2) 27.6% (4) 62.1%

1/05

9 What is the percent composition by mass of aluminum in Al₂(SO₄)₃ (gram-formula mass = 342 grams/mole)?

                (1) 7.89% (2) 15.8% (3) 20.8% (4) 36.0%

69 Show a correct numerical setup for calculating the percent composition by mass of oxygen in NO₂.

6/05

39 A sample of a substance containing only magnesium and chlorine was tested in the laboratory and was found to be composed of 74.5% chlorine by mass. If the total mass of the sample was 190.2 grams, what was the mass of the magnesium? 

                    (1) 24.3 g        (2) 48.5 g          (3) 70.9 g        (4) 142 g 

1/06

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