Ineed find the percentage composition of this compound : ca(oh)2 ,c6h2o , k2so3 , li2so4 ,caco3.

I'm guessing you mean Oxygen, right?

Percent composition by mass is found by finding the total molas mass of the compound and the mass of oxygen and dividing the mass of oxygen by total mass.

1. The molar mass of Ca(OH)2 is 74.0958 amu. There are 2 oxygens, so 16 x 2 = 32 amu. 

32 amu/ 74.0958 amu = 43 % Oxygen

2. C6H2O = 18% Oxygen

3. K2SO3 = 30% Oxygen

4. Li2SO4 = 58% Oxygen

5. This is the only one not having Oxygen but it's 82% Cobalt and 18 % Calcium

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Explanation:

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