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How many hydrogen atoms are in each formula unit of calcium bicarbonate ##Ca(HCO_3)_2##?
two
Looking closely at the part ##(HCO_3)_2##.
In the bracket there is one hydrogen. But outside the bracket, there is a number 2.
It means the number of hydrogen inside the brackets ##xx## 2: [H ##xx## 2] = ##H_2##. Therefore there are two hydrogen atoms present.