Using concentrations of solutions in mol/dm3 (3.4) (M)

Question 1

25.0cm³ of HCl, of concentration 1.0 mol/dm³ reacts with 20.0cm³ of NaOH, solution.

Calculate the concentration of the NaOH solution.

The equation for the reaction is HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Answer 1

• volume of HCl = 25.0 cm³ = 25.0 ÷ 10³ = 0.025 dm³
• volume of NaOH = 20.0 cm³ = 20.0 ÷ 10³ = 0.02 dm³

moles of HCl = 0.025 x 1 = 0.025mol

note: if the number in front of the substance is not 1, you will have to multiply the amount of moles for that ratio e.g. ratio of 2:3, divide by 2 and times by 3; ratio of 1:3, multiply amount of moles by 3)

concentration of NaOH = 0.025 / 0.02 = 1.25mol/dm³

Question 2

How would you covert to g/dm³ when given the concentration in mol/dm³?

Answer 2

Multiply mol/dm³ by Mr

Question 3

Why should phenolphthalein indicator be used for this titration (with vingegar) instead of methyl orange?

Answer 3

Titration of weak acid and strong base