Molecular mass relative (Mr) calculated from the atomic mass number (Ar) of the elements in the molecule. Ar express the weight of the element relative to 1/12 atomic mass of C-12 isotope. Here is the example of the Mr NaCl calculation.

Since Ar Na=23 and Ar Cl=35,5
so, Mr NaCl is equal to the sum of that:
Mr NaCl = (1 x Ar Na) + (1 x Ar Cl) = (1 x 23) + (1 x 35,5) = 23 + 35,5 = 58,5

Now, you have to able to calculate the amount of Mr H2SO4 while known that Ar H = 1, Ar S = 32 and Ar O = 16. Right, the answer below.
Mr H2SO4 = (2 x Ar H) + (1 x Ar S) + (4 x Ar O) = (2 x 1) + (1 x 32) + (4 x Ar 16) = 2 + 32 + 64 = 98

The value of atomic mass number of some elements shown below.
H = 1        Li = 7        Mg = 12
C = 12      Na = 23      Ca = 40
O = 16      K = 39        Fe = 56
N = 14      Cl = 35,5    Cu = 63,5Source:
http://gundulshare.blogspot.com/2012/12/calculate-molecule-mass-relative-of.html

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