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Limiting Reactant Mole Ratios

Posted: Fri Sep 30, 2016 9:30 pm
by Nicole Gamboa 2M
Hi! I just had a general question when it comes to comparing the mole ratios in a limiting reactant equation. Once you've balanced the equation and converted everything into moles, when you compare the two, how do you know which product you compare the actual ratio with?

For example, in the equation CH4 + O2 --> CO2 + H2O, where you have 30g of CH4 and 15g of O2, after you've balanced it (CH4 +2O2 --> CO2 + 2H2O) and converted it to moles (1.87mol CH4 and .4690mol O2), how do you know whether to compare the moles you just calculated to the moles of CO2 or H2O?

Re: Limiting Reactant Mole Ratios

Posted: Fri Sep 30, 2016 10:02 pm
by Geoffrey Zhang 3I
I didn't really follow your question, but I'll walk through the example you gave. So after you converted to moles (1.87 mol CH4 and 0.4690 mol O2) and balanced your equation, you need to figure out which reactant is your limiting reagent. Based on your balanced equation, you need 2 mol of O2 for every 1 mol of CH4, and in your calculated moles, you have only 0.4690 mol O2 for 1.87 mol mol CH4. So, your limiting reagent is O2. From there, you use 0.4690 mol O2 in your calculation to find whatever the question is asking.So if the question asks how many grams of CO2, compare the mole ratios of O2 and CO2 which is 2 mol O2/1 mol CO2. If the question asks for H2O, compare 2 mol O2/2 mol H2O. Hopefully that answers your question.