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
