Question

Suppose you begin with 1.50~g of the hydrate copper(II)sulfate · x-hydrate (CuSO4· x H2O), where x is an integer. After dehydration you find that you are left with 0.96~g of the an-hydrate CuSO4. What is the unknown integer x. Round your answer to the nearest integer, enter only an integer.
Suppose you begin with of the hydrate KAl(SO4)2 · 12H2O. After dehydration you find that you are left with 3.0~g of the an-hydrate KAl(SO4)2. How many grams did you start with? Write the value to the correct number of significance figures.

Answer 1

These are two questions and two answers.

Question 1.

Answer: x = 5

Explanation:

1) Data:

m₁ = 1.50 g

compound₁: CuSO₄· x H₂O

m₂ = 0.96g

compound₂ = CuSO₄

x = ? (round to the nearest integer)

2) Solution:

i) molar mass of CuSO₄ = 63.546g/mol + 32.065g/mol + 4×15.999g/mol = 159.607g/mol

ii) number of moles of CuSO₄

number of moles = mass in grams / molar mass = 1.50 g/ 159.607 g/mol = 0.006265 mol

iii) molar mass of H₂O = 18.015 g/mol

iv) mass of H₂O = 1.50g - 0.96g = 0.54g

v) number of moles of H₂O = mass in grams / molar mass = 0.54 g / 18.015 g/mol = 0.0300 mol

vi) Ratio moles H₂O / moles CuSO₄ = 0.0300 / 0.0062625 ≈ 5

∴ x = 5.

Question 2.

Answer: 5.5 g

1) Data:

compound₁ = KAl(SO₄)₂ · 12H₂O.

compound₂ = KAl(SO₄)₂

m₂ = 3.0 g KAl(SO₄)₂

m₁ = ? (two significant figures)

2) Solution:

i) molar mass of KAl(SO₄)₂ = 39.098g/mol + 26.982g/mol + 2×32.065g/mol + 8×15.999g/mol = 258.202

ii) number of moles of KAl(SO₄)₂ = mass in grams / molar mass = 3.0g / 258.202 = 0.011619 mol

iii) number of moles of H₂O = 12 × number of moles of KAl(SO₄)₂ = 12 × 0.011619*12 mol = 0.1394 moles

iv) mass of H₂O = number of moles × molar mass = 0.1394 moles × 18.015 g/mol = 2.5 g (rounded to two significant figures)

v) mass of the original compound = mass of KAl(SO₄)₂ + mass of H₂O = 5.5g