You have a powder that dissolves in water to form a strong acid. You dissolve 2g of the powder in 100ml of water, and measure the pH to be 4. You would like to form 100ml of a solution with pH 5. How many grams of the powder do you dissolve in 100ml of water?

0.2 g

Explanation:

pH = - log (H⁺)

If the pH is 4 then

4 = - log (H⁺)

10⁴ = (H⁺) ⁻¹

(H⁺) = 1 / 10⁴ = 0.0001 M

for pH 5

5 = - log (H⁺)

(H⁺) = 1 / 10⁵ = 0.00001 M

if 2 g in 100ml yielded 0.0001 M

then x g in 100 ml yielded 0.00001 = 2 * 0.00001 / 0.0001 = 0.2 g

The mass of the powder to dissolve in 100ml of water to form a solution with pH 5 is 0.2 g

Explanation:

pH = - Log [H⁺]

when the pH of the solution = 4,

the concentration is calculated as follows

10⁻⁴ = [H⁺] = 1 X 10⁻⁴ M/L

when the pH of the solution = 5,

10⁻⁵ = [H⁺] = 1 X 10⁻⁵ M/L

Also concentration, = number of moles/liters of solution

number of moles = Reacting mass (Rm) / Molar mass

Concentration is directly proportional to reacting mass

Thus, C₁/Rm₁ = C₂/Rm₂

Rm₂ = (C₂Rm₁) / C₁

Rm₂ = (1 X 10⁻⁵ * 2g) / (1 X 10⁻⁴)

Rm₂ = 0.2 g

Therefore, the mass of the powder to dissolve in 100ml of water to form a solution with pH 5 is 0.2 g