[findingneverland]

A 0.4117 g sample of pesticide preparation was transferred to a flask containing 50 mL of 0.0996 M NaOH and 50 mL of 3% H2O2. Heating caused oxidation of formaldehyde according to the reaction
OH- + HCHO + H2O2 -> HCOO- + 2H2O
The solution was cooled and the excess base was titrated with 19.66 mL of 0.0550M H2SO4. Calculate the percentage of formaldehyde in the sample.

I got the right result for this problem, i'm only wondering why do we take only the moles of NaOH and don't include H2O2 in the calculation?

[Hunter2]

How do you want include it.

The sodium hydroxide is consumed and the Peroxide as well. You have to pick only one chemical what you trace. NaOH its easy to determine. If you know the beginning concentration of peroxide and then after the reaction took place the remaining amount you can calculate with this as well.