A sample of 0.3283 g of an ionic compound containing the bromide ion (Br-) is dissolved in water and treated with an excess of AgNO3. If the mass of the AgBr precipitate that forms is 0.7127 g, what is the percent by mass of Br in the original compound?

92.49 %

Explanation:

We first calculate the number of moles n of AgBr in 0.7127 g

n = m/M where M = molar mass of AgBr = 187.77 g/mol and m = mass of AgBr formed = 0.7127 g

n = m/M = 0.7127g/187.77 g/mol = 0.0038 mol

Since 1 mol of Bromide ion Br⁻ forms 1 mol AgBr, number of moles of Br⁻ formed = 0.0038 mol and

From n = m/M

m = nM. Where m = mass of Bromide ion precipitate and M = Molar mass of Bromine = 79.904 g/mol

m = 0.0038 mol * 79.904 g/mol = 0.3036 g

% Br in compound = m₁/m₂ * 100%

m₁ = mass of Br in compound = m = 0.3036 g (Since the same amount of Br in the compound is the same amount in the precipitate.)

m₂ = mass of compound = 0.3283 g

% Br in compound = m₁/m₂ * 100% = 0.3036/0.3283 * 100% = 0.9249 * 100% = 92.49 %