If 5.00 mL of 3% H2O2 is decomposed, how many mL of gas will be produced at 1.01 bar and 25C?

Answer 1

3% what? w/v. That's what I will assume.
3% w/v means 3 g H2O2 per 100 mL solution.
2H2O2 ==> 2H2O + O2

In the 5.00 mL you will have
3g x (5/100) = ? g H2O2

Convert to mols; mols = grams/molar mass = ?
Using the coefficients in the balanced equation convert mols H2O2 to mols O2 produced.
Then use PV = nRT an solve for V (in L) at the conditions listed and convert to mL.