Question

1. Explain how Na2CO3 of primary standard grade can be prepared from primary standard grade NaHCO3? 2. Why is it common practice to boil the solution near the equivalence point in the standardization of acid with Na2CO3? 3. Calculate the molar concentration of a dilute HCl solution if titration of 0.2694 g of primary standard Na2CO3 required 38.77 mL of the acid (products CO2 and H2O).

Answer 1

Question 2.

NaHCO₃ on reaction with acid liberates CO₂. If the solution is not boiled at equivalence point, the CO₂ liberated in the reaction may remain dissolved in the water and form a weak Carbonic acid which will thereby make the solution slightly acidic instead of neutral.
Na₂CO3 + HCl = NaCl + CO₂ + H₂O
CO₂ + H₂O = H₂CO₃

Question 1.

Primary standard Na₂CO₃ can be obtained by heating primary standard grade NaHCO₃ for about an hour at high temperatures (300 °C). When this happens, The reaction followed is:

2NaHCO₃(s) ----------> Na₂CO₃(_g) + H₂O(_g) + CO₂(_g)

Question 3.

The reaction is as follow:

Na₂CO3 + HCl -----> NaCl + CO₂ + H₂O

We have a 1:1 relation between the prymary standard and HCl so:

moles of Na₂CO₃ = moles HCl

MW Na₂CO₃ = (23*2) + 12 + (3*16) = 106 g/mol

moles of Na₂CO₃ = 0.2694 / 106 = 2.5415x10^-3 moles

M = moles / V

M = 2.5415x10^-3 / 0.03877 = 0.0655 M

Hope this helps