How many moles of nitrogen are in 66.0 g of nitrous oxide?

Rmm of N20 = (14X2) + 16 = 44
Percentage by mass of Nitrogen in N20 = 28/44x100 = 63.64
66x0.6634 = 43.7844g of Nitrogen in 66g of N20
Moles = Mass/Mr = 43.7844/14 = 3.13 moles

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Rate law for this reaction israte of the reaction = k[a]²[b].second order in a  means concentration of a in power of two, and  first order in b means concentration of b in power of one.