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Daily Dose 007 | CHEMISTRY
How do you find the number of moles of a substance?
What’s your first thought when you read “find the number of MOLES of a substance”?
If reading the word MOLE makes you think first of a little rascal that digs holes in the backyard, welp, there is some work that we need to do in getting prepared for the FE Exam.
And that’s OK, because moles – in the animal world – can be cute at times 🙂
Being asked to find the number of MOLES of a substance may seem complicated at first, but understanding that the process can be simplified (and replicated across the most complex scenarios) will ease any potential negative anticipation to seeing this problem type on your exam.
In this lesson (and video), we dive in to a FE Exam Practice Problem, this time in the subject of CHEMISTRY, specifically revolving around how to find the NUMBER OF MOLES of a substance in a given sample.
Key Definition
What is a MOLE?
The MOLE is a unit of measurement used in CHEMISTRY.
It measures the amount of pure substances containing the same number of chemical units as there are atoms in exactly 12 grams of carbon-12.
However, we don’t use the 12 grams of carbon-12 much on the FE Exam, but rather, AVOGADRO’S NUMBER, which is:
- 1 mol = 6.023e23 particles
Initially, the connotation of MOLE was “gram molecular weight”, however, the usage tends to apply the term MOLE to an amount containing Avogadro’s number of whatever units are being considered. For this reason, it is possible to have a MOLE of atoms, ions, radicals, electrons, particles, or quanta.
In the NCEES FE Reference Handbook, we see the units of PARTICLES used, but this can in the same way say ATOMS, IONS, etc.
The Process
How can we find the number of moles of a substance?
Finding the number of moles of a substance may seem intimidating at first, but the following steps will allow you to quickly and efficiently solve this type of Chemistry problem on the FE Exam:
- Step 1: Identify the Substance and its Formula
- Step 2: Determine the Molar Mass
- NaCl = 22.99 + 35.45 = 58.44 g/mol.
- Step 3: Use the Given Mass
- Step 4: Divide the given mass by the molar mass of the substance
- # of Moles = 117 g/58.44 g/mol = 2 Moles
- Step 5: Choose (report) the Result
Reading through the problem statement, determine the chemical formula of the substance in question. For example, if you’re asked about sodium chloride, the chemical formula is NaCl.
Using the periodic table, find the atomic masses of each element in the substance.
Once those are defined, multiply the atomic mass of each element by the number of atoms of that element in the defined substance.
Sum up the results to get the molar mass (also known as molecular weight) of the substance. The unit will be grams/mole (g/mol).
As an example, for NaCl, we use the periodic table to determine that the atomic mass for Na is 22.99 g/mol and the atomic mass for Cl is 34.45 g/mol.
There is a single atom of each in NaCl, therefore, summing them together, we find that the molar mass of NaCl is:
The problem will typically provide you with a given mass of the substance – note this value.
Continuing with our example, say the problem states that you have 117 grams of NaCl.
Having the Molar Mass of a NaCl defined, we can determine the number of MOLES as:
Present your answer with the correct number of significant figures (based on the data provided in the problem statement) and with the appropriate units, which will be MOLES.
Here are some additional tips to consider when being asked to find the number of moles of a substance:
- Always make sure your units are consistent. Typically, you’ll want everything in grams and g/mol.
- Remember that molar mass values might slightly vary depending on the source (i.e., different periodic tables or textbooks), so always use the values provided in the NCEES FE Reference Manual or the values specified in the problem statement.
- Practice with diverse problems to become more familiar with the process and potential variations in the type of problems.
By following these steps, you should be able to effectively calculate the number of moles of a substance no matter how complex the problem statement becomes.
Now check out the video and see how to find the number of moles in a sample in the most efficient manner come your FE Exam day.
As always, we are here to help, Prepineer
