To calculate the percent yield of a reaction, you have to calculate limitant and excess first of the reaction:
6CO2 + 6H2O + light -> C6H12O6 + 6O2
We have 18.6 moles of CO2 and 238g of H2O
Finding the number of moles of water will help us find the limitant of the reaction
nH2O = mH2O/Molar mass of H2O
nH2O = 238g/18.01g/mol
nH2O = 13.2 moles
So if we have 6 moles of water we will end up with 1 mole of C6H12O6, when we have 13.2 moles, we will have 2.2 moles of C6H12O6
6moles = 1 mol
13.2 moles = x
x = 2.2 moles
The molar mass of C6H12O6 is 180.156 for each mole, since we have 2.2 moles, the final mass will be 396.34 grams
To find the percent yield we have to calculate the actual yield divided by the theoretical yield and then multiply by 100
% yield =( 30g/396.34g) * 100
% yield = 7.6%