What mass of ammonia is necessary to react with 2.1x10^24 molecules of oxygen in 4nh3+7o2=6h2o+4no2 * show steps*

First of all, we need to know how many moles of oxygen 2.1*10^24 molecules of oxygen are. According to the definition of Avogadro's number, 6.02*10^23 molecules are 1 mole. Thus (by proportion) the given amount is equal to: moles of oxygen (O2). By the stoicheiometry of the reaction, for every 7 moles of oxygen reacting, we need 4 moles of ammonia. Hence, we need 4/7*3.49 moles = 2 moles of ammonia. We need to calculate its weight; we know that the atomic mass of hydrogen is 1 and that the atomic mass of nitrogen is 14 (atomic and molecular masses are measured in grams). Hence, the molecular mass of ammonia (that contains 3 hydrogen atoms and a nitrogen atom) is: 3*1+1*14= 17 gr. The molecular mass of a substance is how much one mole of molecules of this substance weighs. Hence, the 2 required moles of ammonia weigh 2*17=34 gr. 34 gr of ammonia is the necessary quantity.