If #"25 cm"^3# of a solution of calcium hydroxide with a concentration of #"1 g/dm"^3# takes a titre of 25 cm^3 of hydrochloric acid, what is the molar concentration of the #"HCl"#?
1 Answer
I suspect that you diluted a stock solution of HCl and used this to titrate the limewater. If that is is correct, there are 3 × 10⁻⁴ mol of Ca(OH)₂ in a 25 cm³ pipette.
If a 25 mL of the Ca(OH)₂ solution takes a titre of 25 cm³ of HCl, the concentration of HCl must be 0.03 mol·dm⁻³.
Step 1. Calculate the molar concentration of the limewater.
(1 significant figure + 1 guard digit)
Step 2. Calculate the moles of Ca(OH)₂ in your sample.
(1 significant figure)
Step 3. Write the balanced chemical equation for the neutralization.
Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
Step 4. Calculate the moles of HCl required.
(1 significant figure + 1 guard digit)
Step 5. Calculate the molarity of the HCl.
Molarity =
(1 significant figure)
Note: The answer can have only 1 significant figure, because that is all you gave for the concentration of the limewater. If you need more precision, you will have to recalculate.