My working:
i found the mole of cr2+ ions in diluted 25 cm^3 solution by dividing the original moles of chromium (||) ethanoate with 10 = 2.66x10^-4 mol
then i took 1 equation from the booklet for Mn where it changes its oxidation no. from 7+ to 2+
and wrote a balanced equation of Cr2+ getting oxidised to 6+.[cr2+ -----> Cr6+ + 4e-]
after both half equations: (final redox equation)
4MnO4- + 32H+ + 5Cr2+
-----> 4Mn2+ + 16H2O + 5Cr6+
the molar ratio of Mn7+ and Cr2+ in titration is 4:5
so, to get the vol. of KMnO4 required in titration:
[ (4×2.66×10^-4)/5 ] ÷ 0.0075 = 0.02837 dm^3
so it's giving me, 28.37 cm^3 as the final answer.
but the ms says the answer is: 56.75cm^3
HOW??
i'd be grateful if you can point where i went wrong or what i missed out.
thank you in advance!!