Question

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How many grams of waster are produced in burning 2.24dm' of hydrogen at STP?​

Answer 1

You start with 0.1 mole of hydrogen burn it to get water. The formula of water is  H2O, So you get 0.1 mole of water.  The molecular weight of water is 18. 0.1 mole of water would then weight 1.8 g

There are three rules to solve that kind of problem:  

At STP a mole of perfect gas has a volume of 22.4  dm3  

A mole is an amount of molecules. It is enough molecules of a substance to get its molecular weight in gram.

Hydrogen can be assumed to be a perfect gas*

So 1 mole of  H2 has a volume of 22.4  dm3 at STP. A molecule of hydrogen is made of 2 atomes of hydrogen and thus has a molecular weight of 2. It follows that a mole of  H2 has a weight of 2 g.

Answer 2

Explanation:   as given that 2.24dm cube of hydrogen so first you will have to find the number of moles of hydrogen by diving 2.24dm cube by 22.4dm cube where you will get 0.1mol of hydrogen. as we have 2 moles of hydrogen and 2 moles of water in the balanced chemical equation so first multiply 0.1 by 2 so you will get 0.2 moles of hydrogen and now divide 0.2 moles of hydrogen by 2 moles of water so you will have 0.2 moles of water . now to find its mass in grams multiply 0.1 moles of water by its molar mass (18g/mol) so you will get the mass in grams of water produced by burning 2.24dm cube of hydrogen at STP.