A 5.30 g sample of an alkali metal hydroxide was dissolved in water - HSC - SSCE Chemistry - Question 28 - 2021 - Paper 1

Given that the mass of the precipitate (Cu(OH)₂) is 4.61 g,

We can find the moles using the molar mass of Cu(OH)₂:

1. Calculate the Molar Mass of Cu(OH)₂:

   • Molar mass of Cu = 63.55 g/mol
   • Molar mass of O = 16.00 g/mol
   • Molar mass of H = 1.01 g/mol
   • Therefore, $\text{Molar mass of Cu(OH)}_2 = 63.55 + 2(16.00 + 1.01) = 97.57 \, \text{g/mol}$

2. Find Moles of Cu(OH)₂: $\text{Moles of Cu(OH)}_2 = \frac{\text{Mass}}{\text{Molar Mass}} = \frac{4.61 \, \text{g}}{97.57 \, \text{g/mol}} = 0.0472 \, ext{mol}$

From the balanced equation, 2 moles of OH⁻ react with 1 mole of Cu(OH)₂, therefore:

1. Calculate Moles of OH⁻: $n_{\text{OH}^-} = 2 \times n_{\text{Cu(OH)}_2} = 2 \times 0.0472 \, \text{mol} = 0.0944 \, \text{mol}$

Using the total mass of the alkali hydroxide sample (5.30 g) and the moles of OH⁻ calculated:

1. Calculate the mass of OH⁻: $\text{Mass of OH}^- = n_{\text{OH}^-} \times \text{Molar Mass of OH}^- = 0.0944 \, \text{mol} \times 17.01 \, \text{g/mol} = 1.604 \, \text{g}$

2. Mass of Alkali Metal: $\text{Mass of Alkali Metal} = 5.30 \, \text{g} - 1.604 \, \text{g} = 3.696 \, \text{g}$

3. Identifying the Alkali Metal:

   • Using periodic table values to identify the alkali metal (Na, K, etc.), the closest match corresponds to Potassium (K).

Therefore, the alkali metal hydroxide is Potassium Hydroxide (KOH).