# Can you help me balance this equation? As2S3 + K2Cr2O7 + H2SO4 = H3AsO4 + K2SO4 + Cr2(SO4)3 + H2O + SO2

$A {s}_{2} {S}_{3} + {K}_{2} C {r}_{2} {O}_{7} + {H}_{2} S {O}_{4} \implies {H}_{3} A s {O}_{4} + {K}_{2} S {O}_{4} + C {r}_{2} {\left(S {O}_{4}\right)}_{3} + {H}_{2} O + S {O}_{2}$

So, count the number of each atoms on both sides. To ensure a chemical equation is fully balanced, each atoms on both sides must be equal.

LEFT of equal sign:
As = 2
S = 3
K = 2
Cr = 2
O = 11
H = 2

RIGHT of equal sign:
As = 1
S = 5
K = 2
Cr = 2
O = 23
H = 5

At this stage, only K and Cr are balanced. This is a very complex equation, so it takes some time to solve it.

First step, find the atom that is easy to balance first.
I would say As because you have 1 As atom on the right but 2 on the left, so look for the compound where it has an As on it (on the RIGHT side) and add a 2 in front of it.
It should look like $A {s}_{2} {S}_{3} + {K}_{2} C {r}_{2} {O}_{7} + {H}_{2} S {O}_{4} \implies 2 {H}_{3} A s {O}_{4} + {K}_{2} S {O}_{4} + C {r}_{2} {\left(S {O}_{4}\right)}_{3} + {H}_{2} O + S {O}_{2}$.

Every time you make a change in balancing an equation, keep checking how many atoms on the left and right of equal sign. It's basically doing a trial-and-error method.

Next try.

LEFT of equal sign:
As = 2
S = 3
K = 2
Cr = 2
O = 11
H = 2

RIGHT of equal sign:
As = 2
S = 5
K = 2
Cr = 2
O = 26
H = 8

As, K and Cr is balanced at the moment. Look for the next atom to balance. I would say H. On the left, there are 2 H atoms and right 8 H atoms. In order for the H on the left hand side to get 8 H atoms, add a 4 in front of the atom that has the H in it. Looking at the equation, the H is part of ${H}_{2} S {O}_{4}$ so add a 4 in front of the ${H}_{2} S {O}_{4}$.

Note that an atom could usually also be part of multiple compounds.

Anyway, $A {s}_{2} {S}_{3} + {K}_{2} C {r}_{2} {O}_{7} + 4 {H}_{2} S {O}_{4} \implies 2 {H}_{3} A s {O}_{4} + {K}_{2} S {O}_{4} + C {r}_{2} {\left(S {O}_{4}\right)}_{3} + {H}_{2} O + S {O}_{2}$

LEFT of equal sign:
As = 4
S = 3
K = 2
Cr = 2
O = 23
H = 8

RIGHT of equal sign:
As = 2
S = 5
K = 2
Cr = 2
O = 26
H = 8

Keep doing same thing on and on until the atoms on the left are equal to the right. If the number of atoms on the left = number of atoms on the right, then your equation is balanced.