[aaronstonedd]

The numerical in it's full glory:
10 mL (sic) of hydrogen combine (sic) with 5 mL (sic) of oxygen to yield water. When 200 mL (sic) of hydrogen at NTP are (sic) passed over heated CuO, the latter loses 0.144 g of its mass. Do these results agree with the law of constant composition?

• Do hydrogen and oxygen refer to
  (i) "H" and "O", or
  (ii) "H₂ and O₂, or
  (iii) "H₂ and O?
• What results are we supposed to expect, that can clarify whether "these results" agree with the law of constant proportions?
  What is the step-by-step procedure we have to follow to get "the results ... we are supposed to expect"?

[Borek]

H₂ (sic) and O₂ (sic).

Apparently you should check if the oxygen/hydrogen proportion is identical in each case (sic).

[opsomath]

Dear OP, I think you have fallen afoul of Muphry's Law in your use of "it's" and your many, many uses of "sic".

It isn't correct to ask the internet for a step by step procedure to figure out your results; part of the question is whether you can figure that out, I think.

So, let me ask you this in turn; what chemical reaction is going on here? Write its balanced equation.

[Arkcon]

I think you can assume that H₂ and O₂ are being used.  These gasses are diatomic, we can't "pass over" or "combine" H and O.  Those may be used to visualize reactions, but they're not things we can measure.  A tank full of hydrogen or oxygen, we can measure those as full or empty, and they'll contain diatomic gasses.