Question

For the next three questions, use the following scenario: You discover an unlabeled solution of Mg(OH)2 on your work bench and decide to use a solution of 1.25 M HCl for your titration process. You placed 35 mL of HCl into the flask and used 15 mL of Mg(OH)2 from the buret to complete the titration.

Calculate the concentration of your unknown solution. You MUST show all work to receive full credit

Answer 1

1) State the balanced chemial reaction

Mg (OH)2 + 2 HCl → Mg Cl2 + 2 H2O

2) State the molar ratios

1 mol Mg(OH)2 : 2 mol HCl : 1 mol MgCl2 : 2 mol H2O

That means that 2 mol of HCl react (neutralize) 1 mol of Mg (OH)2.

3) Calculate the number of moles of HCl from its molar concetration

M = number of moles / volume in liters = n / L

35 mL of HCl 1.25 M => n = 1.25 M * 0.035 L =0.04375 mol HCl

4) Calculate the number of moles of Mg (OH)2 from the molar ratio

0.04375 mol HCl * [1 mol Mg (OH)2 / 2 mol HCl] = 0.021875 mol Mg(OH)2.

5) Calculate the Molarity using M = n / V in liters

M = 0.021875 mol / 0.015 L = 1.46 M

Using two significant figures M = 1.5 M

Answer: 1.46 M or 1.5 M depending on the number of significant figures.