Acid-based titration calculation

I'm stuck on this practice question for A-level Chemistry:

An indigestion tablet containing sodium hydrogencarbonate was dissolved in 250cm^3 of water. 25.0cm^3 of this solution was found to need 16.50cm^3 of a 0.150mol dm^-3 solution of HCl for neutralisation.

NaHCO3+2HCl --> NaCl + H2O + CO2

If the original mass of the indigestion tablet was 3.20g, calculate the percentage by mass of sodium hydrogencarbonate in the tablet.

I know the correct answer is 32.5% but I keep getting 26% for some reason. Could someone show me how to work it out?
Any help would be much appreciated.

show us how you get 26% and we'll point out your error ...

I'm stuck on this practice question for A-level Chemistry:

An indigestion tablet containing sodium hydrogencarbonate was dissolved in 250cm^3 of water. 25.0cm^3 of this solution was found to need 16.50cm^3 of a 0.150mol dm^-3 solution of HCl for neutralisation.

NaHCO3+2HCl --> NaCl + H2O + CO2

If the original mass of the indigestion tablet was 3.20g, calculate the percentage by mass of sodium hydrogencarbonate in the tablet.

I know the correct answer is 32.5% but I keep getting 26% for some reason. Could someone show me how to work it out?
Any help would be much appreciated.

An indigestion tablet containing sodium hydrogencarbonate was dissolved in 250cm^3 of water. 25.0cm^3 of this solution was found to need 16.50cm^3 of a 0.150mol dm^-3 solution of HCl for neutralisation.

NaHCO3+HCl --> NaCl + H2O + CO2

If the original mass of the indigestion tablet was 3.20g, calculate the percentage by mass of sodium hydrogencarbonate in the tablet.

This is my working:

.........NaHCO3 + HCl
Conc................0.150
Vol..................0.0165
Moles 0.002475....0.002475


Ratio 1:1

16.50cm^3 / 1000 = 0.0165 dm^3
0.150 x 0.0165 - 0.002475 moles


.......NaHCO3
Mass 0.2079
Mr 84
Moles 0.002475

Mr = 23 + 1 + 12 + 3(16) = 84
0.002475 x 84 = 0.2079g

0.2079g / 0.250 = 0.8316

(0.8316/3.2) x 100 = 25.9875%
= 26.0% (3 sf)

I've never done this type of calculation before so I don't know where I've gone wrong. Thanks for any help.

Here is the worked solution which gives you % mass of 32.5%.

1. n(H+) titration = (16.5/1000) x 0.15 = 0.002475mol
work out the moles of H+/HCl needed to neutralise the HCO3- ion in the titration.

2. n(HCO3-) = 0.002475 / 2 = 0.0012375mol
look at the mole ratio in the reaction you provided. For every one HCO3- ion there were 2 H+ ions reacting. So we divide the moles of H+ by 2 to get moles of HCO3- in the titration.

3. n(HCO3-) total = 0.0012375 x 10 = 0.012375mol
remember that the mol of HCO3- calculated in step 2 was only in 25cm3 solution. Originally the tablet was dissolved in 250cm3, which is 10 times the volume. So the total amount of HCO3- is 10 times the amount in the titration. (i keep mentioning HCO3- because the NaHCO3 is ionic and will dissociate when in the solution)

4. mass of NaHCO3 = 0.012375 x (23 + 1 + 12 + 48) = 1.0395 g
now we know moles of NaHCO3 ( moles of HCO3- = moles of Na+ in the ionic lattice) and its Mr, we work out its mass

5. % mass = (1.0395 / 3.2) x 100 = 32.484375% which rounds to 32.5%

up to this point you are OK, but you now need to multiply by 10 to get moles equivalent in 250ml

= 0.02475 mol

You then find out the mass of this number of moles of sodium hydrogencarbonate

= 0.02475 x 84 = 2.079g

and now use the actual mass to find the percentage purity

= 100 * 2.079/3.02

=68.9%

The ratio is 1:1

NaHCO₃ + HCl --> NaCl + H₂O+CO₂

Thank you! I got 64.9% when I tried it again (I think you meant 64.9 and not 68.9). Thanks for the help.

actually I meant 68.8%

But 100 * 2.079/3.02 = 64.96875 (which would round up to 65)

But 100 * 2.079/3.02 = 64.96875 (which would round up to 65)

Not on my calculator it's not ...

The mass of the tablet is 3.2g, not 3.02g.
So the % mass is 2.079/3.2 x 100 = 64.9...%

You got 68.8...% because you did 2.079/3.02 x 100

The ratio is 1:1

NaHCO₃ + HCl --> NaCl + H₂O+CO₂

sorry im commenting late but its 2HCl so the ratio isnt 1:1 but 2mol hcL:1mol NaHCO3 so the answer is right

The ratio is 1:1.

what do you mean