How can I do stoichiometry mole to mole problems?
Answers
N2 + 3H2 ---> 2NH3
Example #1: If we have 2.00 mol of N2 reacting with sufficient H2, how many moles of NH3 will be produced?
Comments prior to solving the example
(a) The equation is already balanced.
(b) The ratio from the problem will have N2 and NH3 in it.
(c) How do you know which number goes on top or bottom in the ratios? Answer: it does not matter, except that you observe the next point ALL THE TIME.
(d) When making the two ratios, be 100% certain that numbers are in the same relative positions. For example, if the value associated with NH3 is in the numerator, then MAKE SURE it is in both numerators.
(e) Use the coefficients of the two substances to make the ratio from the equation.
(f) Why isn't H2 involved in the problem? Answer: the word "sufficient" removes it from consideration.
Solution:
1) We will use this ratio to set up the proportion:
NH3
––––
N2
2) That means the ratio from the equation is:
2
––
1
3) The ratio from the data in the problem will be:
x
––––
2.00
4) The proportion (setting the two ratios equal) is:
2 x
–– = ––––
1 2.00
5) Solving by cross-multiplying gives:
x = 4.00 mol of NH3 produced.
Comment: Notice how the ratio-and-proportion is written. Written in this manner:
x 2.00
–– = ––––
2 1
is equally correct. Just make sure to keep the two quantities associated with the NH3 and the two associated with the N2 on the same relative side. (The ChemTeam tends to not write the ratio and proportion in the style of the one just about, so you won't see it any more.
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