Question

40,0 mL of 0,125 M Mg(OH)2 and 150,0 mL of 0,125 M HCl is mixed together. What is the pH of this solution?

Answer 1

we have:

Molarity of H+ = 0.125 M

Volume of H+ = 150 mL

Molarity of OH- = 2*[Mg(OH)2] = 2*0.125 M = 0.250 M

Volume of OH- = 40 mL

mol of H+ = Molarity of H+ * Volume of H+

mol of H+ = 0.125 M * 150 mL = 18.75 mmol

mol of OH- = Molarity of OH- * Volume of OH-

mol of OH- = 0.25 M * 40 mL = 10 mmol

We have:

mol(H+) = 18.75 mmol

mol(OH-) = 10 mmol

10 mmol of both will react

remaining mol of H+ = 8.75 mmol

Total volume = 190.0 mL

[H+]= mol of acid remaining / volume

[H+] = 8.75 mmol/190.0 mL

= 0.0461 M

we have below equation to be used:

pH = -log [H+]

= -log (4.605*10^-2)

= 1.34

Answer: 1.34