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Daily Dose 027 | Chemistry
How do you find molecular weight of a compound
When asked to find the molecular weight of a compound, do you:
- A) Jump right in with confidence?
- B) Fold up and consider moving to the next problem?
For most engineers, problems that fall within the realm of Chemistry can bring a lot of angst.
No longer are we dealing with the tangible matter as a whole, but rather, the atoms, molecules and ions that make up that matter.
But we don’t need to go the route of option B when we hit these types of problems come exam day.
In fact, after running through the processes just once, you will see just how basic they can be.
The FE Exam is designed to stretch us from all angles that relate to the practice of engineering.
And in doing so, it’s expected that we are able to operate at whatever level we are asked in a proficient manner.
This can only happen if you are putting yourself in difficult situations during your FE Exam prep.
At Prepineer, we ensure that our students are prepped and proficient.
And we will do the same for you.
In this lesson, we dive into an FE Exam Practice Problem, this time in the subject of CHEMISTRY.
The focus will be helping you become comfortable with determining the molecular weight of a compound.
Key Definition
What is MOLECULAR WEIGHT?
MOLECULAR WEIGHT is a measure of the SUM of the ATOMIC WEIGHTS of each ELEMENT present in a MOLECULAR, or COMPOUND.
We use this measure in CHEMISTRY to help us in determining the stoichiometry in chemical reactions and equations.
It is commonly abbreviated as MW or M.W. with units of ATOMIC MASS UNITS (AMU or U) or DALTONS (DA).
We will also be asked to find the MOLAR MASS. It’s the same thing, with one difference — the units. For MOLAR MASS, we see gram-moles or grams per mole.
The Process
How can we find the molecular weight of a compound?
You are sure to be asked to find it on your exam.
It may be a complex compound or it may be simple.
Either way, instead of stressing out about it, do this instead – starting with a mental picture.
You can think of a chemical compound like a sports team, where each member contributes their own unique weight. The molecular weight is the total weight of every atom on that team.
Understanding molecular weight is like knowing the combined strength of that team.
In engineering, that combined weight is what lets you figure out how substances react and how much of a material a reaction actually needs.
Here’s the general process when you get this problem type on your FE Exam:
- Step 1: Identify the Compound: Determine the chemical formula of the compound for which you want to find the molecular weight.
- Step 2: List the Elements: List all the individual elements present in the compound.
- Step 3: Count the Atoms: For each element in the compound, count the number of atoms based on the chemical formula.
- Step 4: Find Atomic Weights: Use the periodic table to find the atomic weight of each element present in the compound.
- Step 5: Multiply and Sum: Multiply the atomic weight of each element by the number of atoms of that element in the compound, then sum these values to get the total weight.
- Step 6: Consider Molecular Structure: If dealing with isotopes or molecular structures that affect atomic weight, adjust the calculation accordingly.
- Step 7: Verify the Calculation: Double-check the atomic weights and the count of atoms to ensure the accuracy of your result.
Following these steps will help you find it quick, efficiently and most important, accurately.
Worked Example
Let’s find the molecular weight of sulfuric acid
Take sulfuric acid, H2SO4. The subscripts tell you how many of each atom you have — 2 hydrogen, 1 sulfur, 4 oxygen. Multiply each by its atomic weight and add them up.
- Hydrogen: 2 atoms × 1.01 = 2.02
- Sulfur: 1 atom × 32.06 = 32.06
- Oxygen: 4 atoms × 16.00 = 64.00
Sum them: 2.02 + 32.06 + 64.00 = 98.08 g/mol.
That’s the whole move. Count the atoms from the subscripts, pull the atomic weights, multiply, and add. Every problem of this type on the FE Exam runs the same way — the only thing that changes is the compound.
Check out the video and see how to find it in the most efficient manner come your FE Exam day.
Video Review
[VIDEO] How to find the molecular weight of a compound
Next Step
What’s next?
Molecular weight is a building block. Once you can find it on demand, the problems that sit on top of it stop being intimidating — and on the FE Exam, a lot of them do.
Keep building — finding the mass fraction is the natural next step, and it leans directly on the value you just calculated — 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.
