MCAT Chemistry Question — Nitrous Oxide | Next Step Test Prep MCAT Chemistry Question — Nitrous Oxide | Next Step Test Prep

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Amateur chemists make nitrous oxide at home using this (unbalanced) reaction:

chemical reaction

How many grams of NH4NO3 are required to produce 134.4 L of mixed gas if products are at STP?

(Do NOT heat the ammonium nitrate above 240°C or it may detonate)

 

  1. 160
  2. 240
  3. 960
  4. 1440

 

Explanation

 

This solution has 3 steps: balance the equation, determine the required number of moles of product and reactant, and calculate the mass of reactant necessary. The reaction as written is unbalanced. In the balanced equation, 2 moles of H2O and 1 mole of N2O are made for each 1 mole of NH4NO3. 3 moles of product gas result from each mole of reactant. 22.4 L of gas corresponds to 1 mole of gas at STP, and the specified 134.4 L indicates 6 moles of product gases. 6 moles of mixed product gas requires 2 moles of reactant. NH4NO3 weighs 80 g/mole, so 160 g are required.

 

A) 160, correct. 2 moles of NH4NO3 yields 4 moles of H2O and 2 moles of N2O, for 6 moles of mixed product gas occupying 134.4 L at STP.

 

B) 240, incorrect, This answer results from using the unbalanced equation as written.

 

C) 960, incorrect, This answer results from using the unbalanced equation and calculating the mass of 12 moles NH4NO3 for 6 moles N2O product rather than mixed gas.

 

D) 1440, incorrect, This answer results from using the correct balanced equation and calculating the mass of 18 moles of NH4NO3 for 6 moles of N2O product rather than mixed gas.

 

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