I've calculated and my answer is 1.66 mL.
But I'm not sure bout the ans plus my working is too complicated, can you show me how u get the ans?
Correct me if I'm wrong.
1) to change % to concentration
For H2SO4: 4.74*10/MWH2SO4
= 0.48 mol L-1
For NaOH: 48.5*10/MWNaOH
= 12.13 mol L-1
2) 0.48mol of 21mL H2SO4 is eq to 0.01008 mol
3) since acid to alkali ratio is 1:2, therefore moles of NaOH needed is 0.02016 mol
4) therefore volume needed for 48.5% NaOH is
0.02016*1000/12.13 = 1.66 mL