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Daily Dose 036 | Chemistry
How to solve Mole Fractions using the Molar Ratio?
Being asked to solve mole fractions isn’t something that tends to be at the top of the list of things people want to see on the FE Exam.
However, Chemistry is spec’d, so we’ve got to get to work.
Understanding the concept of the mole fraction will help you perform through the most advanced concepts and principles of Chemistry.
In this video, we dive into a FE Exam Practice Problem in the subject of Chemistry, specifically, dealing with determining the mole fractions given the molar ratio.
Key Definition
What is a MOLE FRACTION?
A MOLE FRACTION is a ratio that represents the molar concentration of a particular substance in relation to the total moles of the mixture as a whole.
The MOLE FRACTION can also be expressed, with a denominator of 100, as a percentage of total moles. It is one way of expressing the composition of a mixture in a dimensionless size, the others being the MASS FRACTION and the VOLUME FRACTION.
The Process
How can I solve Mole Fractions using the Molar Ratio?
To calculate Mole Fractions using the Molar Ratio, you will need the molar amounts or the mole ratios of the different components in a mixture.
The mole fraction of a particular component is the number of moles of that component divided by the total number of moles of all the components in the mixture.
Generally, the following process can be used to calculate Mole Fractions using the Molar Ratio:
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- Step 1: Determine the molar amounts or mole ratios of each component in the mixture.
For example, if you have a mixture of two substances, A and B, with 3 moles of A and 2 moles of B, the molar ratio of A to B is 3:2.
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- Step 2: Calculate the total number of moles in the mixture by adding the molar amounts of all components.
In the example above, the total number of moles is 3 + 2 = 5.
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- Step 3: Calculate the mole fraction of each component by dividing the number of moles of that component by the total number of moles in the mixture.
In the example above, the mole fraction of component A is 3/5 = 0.6, and the mole fraction of component B is 2/5 = 0.4.
Therefore, the mole fractions of the two components in the mixture are 0.6 and 0.4, respectively.
Worked Example
What if I’m given grams instead of moles?
That’s the version you’re most likely to see on the FE Exam, and it adds exactly one step to the front — convert each mass to moles first, then take the ratio.
Say a tank holds 64 grams of methane (CH4) and 88 grams of carbon dioxide (CO2). Find the mole fraction of each.
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- Step 1: Convert each mass to moles by dividing by its molar mass — 16 g/mol for CH4, 44 g/mol for CO2.
64 ÷ 16 = 4.0 moles of CH4, and 88 ÷ 44 = 2.0 moles of CO2.
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- Step 2: Add the moles to get the total.
4.0 + 2.0 = 6.0 total moles.
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- Step 3: Divide each component’s moles by the total.
Mole fraction of CH4 is 4.0 / 6.0 = 0.667, and CO2 is 2.0 / 6.0 = 0.333. They sum to 1.000, so the answer checks out.
Once everything is in moles, mole fractions are always the same move — the part over the whole.
Check out the video and see how we can go about solving this type of problem in the most efficient manner.
Video Review
[VIDEO] How to solve Mole Fractions using the Molar Ratio
Next Step
What’s next?
Mole fractions are one small piece of the chemistry the FE Exam will put in front of you, and knowing how to solve one problem isn’t the same as solving it on demand, under a clock. That gap is the whole job.
Keep building — how to find the molecular weight of a compound is the natural next step — or let us build the plan around you.
We’ll assess where you stand, map a study plan to your timeline, and walk you through to exam day ready to pass. Seven days free, no credit card.
