David W. answered • 03/30/21
Caltech PhD / Stanford Post Doc General Chemistry / Organic Chemistry
This question is asking you to use a balanced equation to determine the amount of a product (water) based on a limiting reagent (oxygen). We are assuming you have enough hydrogen since it was not specified.
To convert between different masses using the equation, you need to convert
grams oxygen --> moles oxygen --> moles water -- > grams water.
You will need a periodic table or know that oxygen is 32 g / mol, water is 18 g/mol.
(O2 = 16+ 16 g / mol O atom, H2O = 16 + 1 + 1 = 18)
Start with oxygen
160 g oxygen / 32 g/mol = 5 moles oxygen
The balanced equation tells you 1 mole of oxygen gives you 2 moles of water, so you will get 10 moles of water, which is 180 g.
5 moles oxygen *( 2 mole water / 1 mole oxygen) * 18 g / mol water = 180 g water.
One of the answers if it's multiple choice will be if you forget the 2 and calculate 5 moles water instead.
Quick check: you have two reactants combining into one product, so you know the answer has to be more than 160 g because there are no other products (all the oxygen goes into water). Another way to do this problem is to say I'm taking 160 g water = 5 moles, and combining it 1:2 with hydrogen, so 10 moles H = 2*5*(1+1) = 20 g hydrogen. Since you are only making water, you will get 160 + 20 = 180 g water. I don't reccomend doing the problem this way though, because sometimes there are multiple products and therefore it's safest to use the formula and convert directly between the two products or reactants through moles and molecular weights.
