Calculate the molar concentration of the acid if 35.18mL of hydrochloric acid was required to neutralize 0.450 g of aluminium hydroxide.

Answer 1

Let's start with a balanced equation.

3 HCl + Al(OH)₃ -> 3 H₂O + AlCl₃

We can see that since aluminum hydroxide has 3 hydroxides per unit, we will need 3 times as many moles of HCl to fully neutralize it.

Next, calculate the moles of Al(OH)₃.

Molar mass = 26.982 + 3x15.999 + 3x1.008 = 78.01 g/mol

0.450 g Al(OH)₃ / 78.01 g/mol = 0.00577 mol Al(OH)₃

Due to the stoichiometry we talked about earlier, we know we must have used 3 times as many moles of HCl as Al(OH)₃. Therefore, the moles of HCl used was:

3 x 0.00577 = 0.0173 moles HCl

Finally, we can calculate the concentration of the acid using the formula:

Molarity = moles / Liters

Molarity = 0.0173 moles / 0.03518 L (note the unit change from mL to L)

Molarity = 0.492 M

The exact decimal may vary depending on how and when you rounded your calculations, but you should have 3 sigfigs in your final answer.