A Level Chem Qn

Pinkbow6826 SG

In a reaction, 25.0 cm3 of hydrogen peroxide solution was reduced to water by 40 cm3 of excess 0.250M iron(II) ions under acidic conditions. 10.0 cm3 of the resultant solution was found to react completely with 13.5 cm3 of 0.020 M potassium managate (VII) solution.

Calculate the concentration of the 25.0 cm3 hydrogen peroxide solution.

UltimaOnline SG

Calculate the concentration of the 25.0 cm3 hydrogen peroxide solution.

In Chem calculations, algebra is your best friend.

Let x be the molarity of H2O2 solution.

Write half-equations and balanced redox equations.

Fe2+ oxidized to Fe3+, and H2O2 reduced to H2O.

Fe2+ oxidized to Fe3+, and MnO4- reduced to Mn2+.

In terms of x, find the moles of H2O2 present.

Apply stoichiometry to find moles of Fe2+ reacted (oxidized) away by the H2O2, in terms of x.

Apply stoichiometry to find moles of Fe2+ reacted (oxidized) away by the MnO4-, in actual numerical value.

Find the total moles of Fe2+ present, ie. moles = molarity x volume.

(moles of Fe2+ reacted by H2O2) + (moles of Fe2+ reacted by MnO4-) = (total moles of Fe2+ present)

Solve for x.

Pinkbow6826 SG

so no of moles of H2O2 in 10.o cm3 resultant soln = no of moles in 25.0 cm3?

UltimaOnline SG

so no of moles of H2O2 in 10.o cm3 resultant soln = no of moles in 25.0 cm3?

No.

In 10cm3 of resulting soln, there is no more H2O2 left. What's present, is the excess Fe2+.

You also have to consider the fact that you're only taking 10cm3 out of 25 + 40 = 65 cm3 resulting solution. After you've found the moles of Fe2+ oxidized / reacted away by MnO4-, you've to multiply this no. of moles by 6.5 times. This will give you the moles of Fe2+ in excess (ie. not oxidized / reacted away by H2O2).

Memorypriest SG

Is the answer 0.173mol/dm^3?

UltimaOnline SG

Is the answer 0.173mol/dm^3?

Hi Memo, Pinkbow6826 may or may not have the school's given answer, and in either case may or may not reply.

There's an effective way to double check your answer, by 'working backwards', ie. assume your answer is given right at the start of the question, and check via calculations that all the other variables' values fit.