030 g of hydrogen chloride HCl is dissolved in water to make

0.30 g of hydrogen chloride (HCl) is dissolved in water to make 2.0 L of solution. What is the pH of the resulting hydrochloric acid solution? 0.90 g of sodium hydroxide (NaOH) pellets are dissolved in water to make 6.0 L of solution. What is the pH of this solution?

Solution

To obtain pH and pOH of aqueous solutions resulting from strong acids and bases, we need to apply the following formulas:

pH = -log₁₀[H⁺] pOH = -log₁₀[OH^-]

Where [H⁺] and [OH^-] are hydronium and hydroxide concentrations, equivalent to the corresponding acid or base molarity due to we are dealing with strong acids and bases.

For the first solution:

0.3 grams HCl * (1 mol / 36.46 grams) = 0.00822 moles

[HCl] = [H⁺] = 0.00822 moles / 2 L = 0.00411 M

pH = -log₁₀[H⁺] = -log₁₀[0.00411 M] = 2.386

For the second solution:

0.90 grams NaOH * (1 mol / 40 grams) = 0.0225 moles

[HCl] = [H⁺] = 0.0225 moles / 6 L = 0.00375 M

pOH = -log₁₀[OH^-] = -log₁₀[0.00375 M] = 2.426

pH = 14 - pOH = 14 - 2.426 = 11.574