A level Chemistry - organic yr 2

Hi there, could someone explain why the reaction:
CH3COCHO + 4[H] -> CH3CHOHCH2OH
needs 4[H].
As in why is it 4 and not 3
Thanks

2 H added to the C=O to make the CH(OH)
2 H added to the CHO to make CH2OH

Thanks but isnt it 2 H added to the C=O to make the CH2?

For the ketone no but for the aldehyde yes

Ketone - one H is added to the C and the other is added to the O to make the alcohol group, which makes the -CH(OH). the carbon has the two other bonds from the CH3- on the left and the -CHO on the right so you can only add 1 H

Aldehyde - the C already has 1 H and another one is added (making the CH2) and the second H is added to the O to form a second alcohol group, making CH2OH

Hope that helps