In these sorts of questions you always need to make sure you're working with numbers (moles) and not masses. That way you can set up some stoich ratios based on the chemical reaction you're given. Start by converting the mass of CO2 to moles using the molar mass given - let's call this n. From the reaction, you can see that for every 12 moles of CO2 produced, 2 moles of C6H6 were consumed; in other words, they react in a 2:12 ratio. Now we can calculate x, the moles of C6H6 required to make the given mass of CO2: 2 C6H6 / 12 CO2 = x C6H6 / n CO2 Solve for x and then use the molar mass of C6H6 to convert back to grams and you have your answer!
Heres how I did this equation (if you havent already gotten the answer) From the information you got from the question, you have 42g of CO2, and you need to find the amount of g of benzene needed to create 42g of CO2. The first course of action to take in questions like this is to convert the CO2 into moles. You do this using the equation n = m/M (n = moles, m = grams, M = molar mass) Mass of CO2 = 42g Molar mass of CO2 (as given to you by the question) 44.01g mol-1 Put that in the equation and you end up with 42g/44.01g mol-1 = 0.9543mol (4 sig.fig) Now you have the mol of CO2, you need to deduce the moles from the benzene. If 2 moles of benzene is used to make 12 moles of CO2, you basically need to divide the number of moles of CO2 by 12 (to get the value of 1) and times it by 2 to get the amount of moles of benzene. Heres the equation for that: 0.9543/12 x 2 = 0.15905mol (This is the amount of moles of benzene) Now you have the moles for benzene, you can work out its mass using this equation: m = M x n (n = moles, m = grams, M = molar mass) Here are the numbers in the equation: 78.1074 (mass of benzene given to you in the question) x 0.15905mol (moles of benzene worked out earlier) = 12.4g (3 sig.fig) In conclusion, the answer is 12.4g. I hope this helped and can be used for future reference!