A 0.90 g of hydrogen chloride (HCl) is dissolved in water to make 5.0 L of solution. What is the pH of the resulting hydrochloric acid solution? Express the pH numerically to two decimal places.
0.50g of sodium hydroxide (NaOH) pellets are dissolved in water to make 4.0L of solution. What is the pH of this solution?
Express the pH numerically to two decimal places.

Explanation:

Mol weight of HCl = 36.5

.90 g of HCl = .90 / 36.5 mole = .02465 mole

HCl      =    H⁺        +      Cl⁻

1 mole     1 mole

1 mole of HCl gives 1 mole of H⁺

.02465 mole of HCl will give .02465 mole of H⁺

.02465 mole of H⁺ in 5 litre soln

concentration of H⁺ =  .02465 / 5 = 4.93 x 10⁻³ M

pH = - log ( 4.93 x 10⁻³ )

= 3 - log 4.93

= 3 - .693

= 2.307

= 2.31

NaOH :

Mol weight of NaOH  = 40

.50 g of HCl = .50 / 40 mole = .0125 mole

NaOH     =    Na⁺        +      OH⁻

1 mole     1 mole

1 mole of NaOH gives 1 mole of OH⁻

.0125 mole of NaOH will give .0125 mole of OH⁻

.0125 mole of OH⁻ in 4 litre soln

concentration of OH⁻ =  .0125 / 4 = 3.125 x 10⁻³ M

pOH = - log ( 3.125 x 10⁻³ )

= 3 - log 3.125

= 3 - .495

= 2.505

pH = 14 - pOH

= 14 - 2.505

= 11.495 .

= 11.50