If 990. mL of water is added to 10.0 mL of 0.10 M HCl, what is the pH of the resultant solution?

Initial no. of moles of HCl = No. of moles of HCl in the resultant solution
M₁V₁ = M₂ V₂
(0.10 M) × (10.0 mL) = M₂ × (999 + 10.0 mL)
M₂ = 0.1 × (10.0/1000) = 0.0010 M

1 mole of HCl dissociates to give 1 mole of H⁺ ions.
In the resultant solution, [H⁺] = [HCl] = 0.0010 M

pH = -log[H⁺] = -log(0.0010) = 3.0

Isn't that amazing -- dilution, adding water, makes the pH bigger!! (Better see why!)

M1V1 = M2V2
M1 = 0.10
V1 = 10 mL
V2 = 1000 Ml

Solve for M2

Answer: 0.0001 M