A freshman chemistry student decides to examine...?

Question

A freshman chemistry student decides to examine the soda fountain methods of making carbonated beverages by measuring the amount of acid in a fixed volume Coca-Cola. He/she decides that he/she will use a 250 ml sample for all of his/her analyses and that he/she will make each batch with a separate CO2 cylinder so that carbon dioxide content in each batch would be as uniform as possible. In a typical experiment, the student saturates the Coke solution with the CO2 in the cylinder. He/she titrates the 250 ml sample to phenolphthalein endpoint with 7.65 ml of a sodium hydroxide solution that contains 5.670 grams of sodium hydroxide per liter. How much carbon dioxide was in the saturated solution?

Answer 1

6.5 g/l.

Answer 2

Moles of the sodium hydroxide solution = 5.670/40=0.141
Molarity = 0.141 mole/L.

Number of moles of NaOH used up = 0.141*7.65=1.08*10^-3 moles
The chemical reaction is
H2CO3 + 2NaOH ---> Na2CO3 + 2H2O.
This means that for every 2 moles of NaOH, we have 1 mole of H2CO3 and hence CO2.

So, number of moles of CO2 = 0.54*10^-4 mole.
(Number of moles in 250mL)
So, number of moles in 1 L = 2.16*10^-3 moles.

In grams/Litre=2.16*10^-3*44
=9.3*10^-2 moles.