What is the molecular formula and the mass of potassium hydrogen sulfate?

To find the correct formula, look at the name of the
compound.

Sulfate means it contains a sulfate group 
(SO4^-2). Because the compound must be charge neutral there has to be an equal number of
positive charges to equal the -2 of the sulfate
group.

Hydrogen has a charge of +1 and potassium also has a
charge of +1 because both are group one elements on the periodic
table.

So the correct formula is: 
KHSO4

To find formula mass, get the mass of each element
from your periodic table and multiply that mass by the number of atoms in the
formula.

for KHSO4:

K =
39.098

H = 1.008

S =
32.06

O = 16 * 4 = 64

Total
mass = 39.098 + 1.008 + 32.06 + 64 = 136.166