(1) Relative molecular mass = sum of relative atomic mass of each element, then the relative molecular mass of citric acid =12× 6+8+16× 7 =192.

(2) The mass fraction of elements = atomic weight of each element × atomic number relative molecular mass × 100%, so the mass fraction of carbon atoms in citric acid is =12×192×100% = 37.5%.

Answer: (1) The relative molecular weight of citric acid is 192.

(2) The mass fraction of carbon atoms in citric acid is 37.5%.