Question

Part A

0.75 g of hydrogen chloride (HCl) is dissolved in water to make 7.0 L  of solution. What is the pH of the resulting hydrochloric acid solution?

Express the pH numerically to two decimal places.

pH =

Part B

0.25 g of sodium hydroxide (NaOH) pellets are dissolved in water to make 8.0 L of solution. What is the pH of this solution?

Express the pH numerically to two decimal places.

pH =

Answer 1

a) molar mass of HCl = 36.5 g/mole

Thus, moles of HCl in 0.75 g of it = mass/molar mass = 0.021

Molarity of HCl solution = moles of HCl/volume of solution in litres = 0.021/7 = 0.003 M

Now, HCl being a strong acid dissociates completely into its respective ions

Thus, HCl(aq) ---------> H⁺(aq) + Cl^-(aq)

Thus, [H⁺]= 0.003 M

pH = -log[H⁺] = 2.532

b) molar mass of NaOH = 40 g/mole

Thus, moles of NaOH in 0.25 g of it = mass/molar mass = 0.00625

Molarity of NaOH solution = moles of NaOH/volume of solution in litres = 0.00625/8 = 0.00078 M

Now, NaOH being a strong base dissociates completely into its respective ions

Thus, NaOH(aq) ---------> Na⁺(aq) + OH^-(aq)

Thus, [OH^-]= 0.00078 M

pOH = -log[OH^-] = 3.107

pH = 14 - pOH = 10.893