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