Answer:
1.67 moles SiC
Explanation:
Given Rxn SiO₂ + 3C => SiC + 2CO
Given mass => 5 moles C => ? moles SiC will be formed
Mass (moles) SiC yield = 1/3(5moles) = 1.67moles SiC
Solution Analysis:
Look at the balanced equation and note the coefficients of the given and unknown. In the above case, the balanced equation shows 3 moles Carbon to 1 mole Silicon Carbide. Since the coefficient for SiC is smaller than the coefficient for C then one should get a yield of SiC smaller than moles of C given. To calculate the yield based on these two compounds make a ratio of coefficients such that when multiplied by the given (5 moles in this case) one obtains a number smaller than 5. That is, 1/3(5 moles) = 1.67 moles SiC. Note that the moles of SiC (=1.67) is smaller than moles of C (=5 moles). You certainly would not get a smaller number than 5 if you used the ratio 3/1(5) moles = 15 moles. Wrong!
Here’s another example:
Given Rxn => 3H₂ + N₂ => 2NH₃
Given 7 moles H₂ calculate the moles of NH₃ produced.
Solution:
Note the known is moles H₂ and NH₃ the unknown. From the balanced equation note the coefficient of NH₃ is smaller than the coefficient for H₂. Therefore using the coefficients of H₂ (3) and NH₃ (2) form a ratio such that when multiplied by the given 7 moles one gets a smaller number (the answer needed).
Therefore, mass (moles) NH₃ produced = 2/3(7) moles NH₃ = 3.33 moles NH₃ produced from 7 moles of H₂. 3.33 < 7 as defined by balanced equation.
Rule of Thumb...
When working equation problems, first convert all given data to moles and then use the ratio of coefficients to make the given smaller or larger as defined by equation. Then, if needed, convert moles to desired dimension for problem given.
