A scientist makes a boric acid (H3BO3) solution by dissolving 22.5 g of H3BO3 in water to make 750.0 mL of solution. Calculate the number of moles of H3BO3 present:

The correct answer is 0.36 mol

Explanation:

First we have to calculate the molecular weight (Mw) of H₃BO₃ from the molar masses of the elements:

Mw (H₃BO₃) = 3 x Molar mass H + Molar mass B + (3 x Molar mass O)

= 3 x 1 g/mol + 10.8 g/mol + (3 x 16 g/mol)

= 61.8 g/mol

The molecular weight indicates that there are 61.6 grams per mol of substance. The scientist has 22.5 g, thus we can calculate the number of moles of H₃BO₃ by dividing the mass into the molecular weight as follows:

Number of moles of H₃BO₃ = mass/Mw = (22.5 g) / (61.8 g/mol) = 0.36 mol

There are 0.36 mol of H₃BO₃ in 750.0 mL of solution.

A = 22.5

B = 61.83

C = 0.364

D = 750.0

E = 0.485