Similarly,
if we are given the mass
of any substance involved in the balanced reaction, we can calculate
the mass of any other
substance in that same reaction.
This will require the use of mole ratios and an equation from the last
half of this unit.
Also,
you will often need to tap into your nomenclature and predicting products skills
for stoichiometry.
Example
If 1.05 g of magnesium metal is
reacted with hydrogen phosphate, magnesium phosphate and hydrogen gas are
produced.
(a) What
mass of hydrogen phosphate is used in the reaction?
(b) What
mass of hydrogen gas is produced?
(c) What
mass of magnesium phosphate is produced?
Answer
Follow
along with the videos, Stoich Problem - Part (a) & Stoich Problem - Part(b), to see how to solve this question.
Pay close attention to the format for these questions – it is non-negotiable, so it is important
to get it right. It will really help you
out in the long run. We will be doing
these kinds of questions for the remainder of the semester. Remember why I am so emphatic about technique and format - I am turning you into chemists that will be able to converse fluently in the accepted science "language." Just like a good language teacher allows creativity of thought, but demands certain modes of communicating those thoughts - science works the same way.
Feel free to quote me on Instagram: "The end result is but one tiny part of the puzzle - the journey to get there is far more important."
Feel free to quote me on Instagram: "The end result is but one tiny part of the puzzle - the journey to get there is far more important."
We
will always use the procedure below to solve these types of questions:
Procedure:
- Create a balanced equation. Be sure to space out the substances to create columns beneath each one.
- In each appropriate column, list out the data provided in the question, for both “knowns” and “unknowns.”
- Determine the number of moles for the “known” substance, using its molar mass and the equation, n = m/M.
- Set
up a mole ratio between the “unknown” and “known” substances, using the coefficients
associated with each of those substances:
nunknown nknown
---------- = ----------
coeff coeff - Using the nunknown, along with its molar mass, determine the mass of the “unknown” substance, using the rearranged equation, m = nM.
- Repeat steps 4 & 5 for any other “unknown” substance for which you desire the mass.
Here
is the fully completed answer:
Homework (SCH 3U): Stoichiometry Problem Set #1-6
Homework (SCH 4C): Stoichiometry Problem Set #1, 3, 4
Student Questions:
1. What's the deal with the SDs/rounding in HW question 1?
Notice that the mass in the question is 400 g, which has 1 SD (significant digit). Thus, carrying the 1 SD through the question, the final answer must have 1 SD.
Also, do not round at each individual step in the calculation. Only round the final answer. It may appear that I am rounding throughout the whole question, if you look at my answer keys, however, I am only rounding at the end. For each calculation I keep all the digits that the calculator spits out to be used in any subsequent calculation. However, since I am lazy, I don't want to write down all these digits. Instead, I only write down what would be the appropriate number of SDs if I was to round that step in the calculation (plus, it helps me to keep track of SDs as I go along in the question). But, once again I am only rounding at the final step in the calculation.
1. What's the deal with the SDs/rounding in HW question 1?
Notice that the mass in the question is 400 g, which has 1 SD (significant digit). Thus, carrying the 1 SD through the question, the final answer must have 1 SD.
Also, do not round at each individual step in the calculation. Only round the final answer. It may appear that I am rounding throughout the whole question, if you look at my answer keys, however, I am only rounding at the end. For each calculation I keep all the digits that the calculator spits out to be used in any subsequent calculation. However, since I am lazy, I don't want to write down all these digits. Instead, I only write down what would be the appropriate number of SDs if I was to round that step in the calculation (plus, it helps me to keep track of SDs as I go along in the question). But, once again I am only rounding at the final step in the calculation.
So, going through question 1:
for KClO3: n = m/M = 400 g/122.55 g/mol = 3.263 973 888 mol (this is the value I use in the next step, even though I show it as 3 mol in the answer key)
mole ratio for O2: n = 3 mol (3/2) = 4.895 960 832 mol (this is the value I use in the next step, even though I show it as 5 mol in the answer key)
for O2: m =nM - 5 mol(32.00 g/mol) = 156.670 746 6 g (now I round and since I'm at the end of the calculation; I need 1 SD, so the answer is 200 g)
for KClO3: n = m/M = 400 g/122.55 g/mol = 3.263 973 888 mol (this is the value I use in the next step, even though I show it as 3 mol in the answer key)
mole ratio for O2: n = 3 mol (3/2) = 4.895 960 832 mol (this is the value I use in the next step, even though I show it as 5 mol in the answer key)
for O2: m =nM - 5 mol(32.00 g/mol) = 156.670 746 6 g (now I round and since I'm at the end of the calculation; I need 1 SD, so the answer is 200 g)
