how do i deduce the moles of the acid reacted with sodium carbonate

Question

The balanced equation is: Na2CO3 + 2HCl ----> 2NaCl + H2O + CO2

Moles of sodium carbonate: 0.03945

average volume of acid added: 26.3 cm3

Acid = HCl

This is standardisation of an acid (titration 1)

Answer 1

From the balanced equation you can see that it takes 2 moles of acid (HCl) to react with each 1 moles of sodium carbonate. So, find the moles of sodium carbonate, and then double it to get the moles of acid needed.

Moles Na₂CO₃ = 0.03945

Moles HCl needed = 0.03945 x 2 = 0.0789 moles HCl needed

Now, if you are actually asking how to find the molarity (M) of the HCl used, then you have...

0.0789 moles HCl/26.3 cm3 x 1000 cm3/L = 3 M