For me, I calculated the mole ratio in the equation. Eg. In
reaction 1, we have 6 mols of gas reactants. Product is 3 mols of
gas substance. So the change is 3 mols.
So do I use 3/6 X 100% = 50%, making this option having the
smallest percentage change?
But for option D, this method is weird. With no gaseous products
formed, do I find percentage change using 3/3 X 100 = 100%
Correct, and yes.
Q2: No. of moles of O = [(8-5.6) / 16 ] = 0.15 mol
So... since 2X + 3O --> X2O3 (this equation is a simplified
form of the idea I am trying to portray, no worries this is in
A-Levels)
No. of moles of X = 0.10 mol
Ar of X = Mass/Moles = 56
So,
No. of moles of X in 5.6g of X = Mass/Molar Mass = 5.6/56 = 0.1
mol
So find the choice that gives 0.1, and that is the answer.
EDIT: dkcx method is a very efficient method, as the mass of X
stated in both scenarios is the same. Take note that if both masses
aren't the same, you'll have to go the long way to avoid confusion.
If you can visualise fast enough though, then please, go ahead and
do the shortcut to save time.