General Chemistry/Titration and pH/Answers

1. 5.00g of NaOH are dissolved to make 1.00L of solution.

a) What is the concentration of H⁺? (8x10-14 mol/L)

Kw = [H+] * [OH-]
[H+] = Kw / [OH-]

The concentration of anything is the amount per unit volume:

[NaOH] = amount / volume

The amount of anything can be found from the mass and the molar mass:

amount = mass / mass per mole (molar mass)

We know the mass and we can find the mass per mole, so substitute amount into the above equation:

[NaOH] = amount / volume
[NaOH] = ( mass / molar mass ) / volume
[NaOH] = mass / ( volume * molar mass )

Since NaOH is a strong base, [OH^-] = [NaOH].

Substitute [NaOH] for [OH^-] in the above [H⁺] equation:

[H+] = Kw / [OH-]
[H+] = Kw / ( mass / ( volume * molar mass )
[H+] = Kw * ( volume * molar mass ) / mass

Kw: 1x10−14 mol2/L2
mass: 5.00g
volume: 1.00L
molar mass: 40.0g/mol (add up the molar masses of Na, O, and H)

[H+] = 1x10−14 mol2/L2 * 1.00L * 40.0g/mol / 5.00g
[H+] = 40x10−14 mol/L * g / 5.00g
[H+] = 8x10-14 mol/L

b) What is the pH? (13.10)

pH = -log10( [H+] / 1M )
pH = -log10( 8x10-14 mol/L / 1 mol/L )
pH = -log10( 8x10-14 )
pH = - ( -13.10 )
pH = 13.10