A 4 g weighed portion of calcium was dissolved in 1 L of water. Calculate the mass fraction

| March 12, 2021 | education

A 4 g weighed portion of calcium was dissolved in 1 L of water. Calculate the mass fraction of calcium hydroxide in the resulting solution.

The mass fraction of a substance is calculated by the formula:

W = m (substance): m (solution) × 100%,

m (solution) = m (substance) + m (water).

m (water) = V p.

m (water) = 1l × 1 = 1 kg = 1000 g.

Let’s find the amount of calcium substance.

n = m: M.

M (Ca) = 40 g / mol.

n = 4 g: 40 g / mol = 0.1 mol.

Let’s compose the reaction equation, find the quantitative ratios of substances.

Ca + 2H2O = Ca (OH) 2 + H2 ↑.

1 mole of calcium accounts for 1 mole of calcium hydroxide. The substances are in quantitative ratios 1: 1. The amount of the substance will be the same.

n (Ca) = n (Ca (OH) 2) = 0.1 mol.

Let us find the mass of Ca (OH) 2.

m = n M.

M (Ca (OH) 2) = 40 + 34 = 74 g / mol.

m = 0.1 mol × 74 g / mol = 7.4 g.

m (solution) = 1000 g + 7.4 g = 1007.4 g.

W = (7.4: 1007.4) × 100% = 0.735%.

Answer: W = 0.735%.



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