Hi,
It's been a very long time since I last balanced equations, but I'll give it a try. Basically you need to make sure quantities are conserved by adding factors in front of your molecules.
For the first equation, you can see that on the right you have :
-One Mn atom : this means MnO2 intervenes only once since it's the only place Mn can come from (unless we decide to add a factor different from 1 in front of it, but for now let's assume it's 1)
-One oxygen atom, but if MnO2 intervenes once, you'll have two of them, so there's going to be a factor two in front of H2O
-Four Cl atoms, so you'll need four times HCl since this is the only molecule it comes from
Now hope the H's are conserved with this reasoning :
MnO2 + 4HCl = MnCl2 + 2 H2O
there are 4 H on the left and 4 H on the right, so the equation is balanced.
For the second one, it's pretty much the same
On the right, if we keep the factor 1 for the first molecule, there are two Na atoms so we can expect 2NaOH on the left
You also want one S so try 1*H2SO4
then give it a try
2NaOH + H2SO4 = 1 * Na2SO4 + ? * H2O
to solve for "?", notice you want 2+4=6 oxygen atoms, so 4 + ? = 6, ?=2
which yields
2NaOH + H2SO4 = 1 * Na2SO4 + 2 H2O
(Notice the H's are conserved : there are 4 of them on each side)
This is essentially trial and error, perhaps somebody more experienced can give more effective methods but guessing is usually not too complicated and does not involve too much maths
This post was edited by Hanako on Oct 12 2018 02:56pm