[Rutherford]

From 20 mg of partially methylated disilane, Si₂H_6-x(CH₃)_x, 27.8 cm³ of hydrogen are evolved during alkaline hydrolysis at 294 K and 97400 Pa. Calculate the degree of substitution x of the methylated disilane. Only Si-H and Si-Si bonds break.

I know to solve this, except one thing, the reaction. For example, the cleavage of the Si-H bond, I thought is:
Si-H+OH^- Si-OH+1/2H₂
while it actually is:
Si-H+H₂O Si-OH+H₂
How could I known that it is the second reaction and not the first, when they said that it is an alkaline hydrolysis, so I expected OH^- ions to participate? Where is the logic behind this?

[Borek]

Si-H+OH^- Si-OH+1/2H₂

It can't be right for a very simple reason.

[Rutherford]

  The charges can't be balanced. I just couldn't notice it. Thanks.