The exam comes back covered in red marks. A mole ratio flipped. A volume left in milliliters. An answer you knew, marked wrong because you read “is NOT” as “is.” Those aren’t knowledge problems. They’re execution problems, and they’re the most frustrating chemistry exam mistakes because you could have had those points. Which mistakes keep costing you easy points, and how do you stop making them?
The most common chemistry exam mistakes are dropping units, rounding too early, typing numbers into the calculator wrong, misreading “NOT” or “EXCEPT” questions, using the wrong mole ratio, mixing up signs in thermochemistry, and spending too long on one hard problem. Making them doesn’t mean you don’t understand chemistry. They mean your process has leaks. Each one has a clear warning sign and a simple habit that plugs it. Fix two or three of them and you can pick up points on your next exam without learning a single new topic.
Dropping Units Halfway Through
This is the big one. If you stop writing units, you lose your only built-in error check.
The warning sign is an answer that looks wildly too big or too small, and you don’t notice. Units would have told you. When every number carries its unit, a wrong setup shows up as units that won’t cancel.
Worked example: a unit error and an early rounding error
Question: How many grams of NaOH are in 25.0 mL of 0.150 M NaOH? (Molar mass of NaOH = 40.00 g/mol.)
The wrong way: 0.150 × 25.0 × 40.00 = 150. g. That’s about the mass of an apple, supposedly dissolved in less than two tablespoons of solution. It’s way off because molarity is moles per liter, and the volume was never converted.
The right way, with units:
- 25.0 mL × (1 L / 1000 mL) = 0.0250 L
- 0.0250 L × 0.150 mol/L = 0.00375 mol NaOH
- 0.00375 mol × 40.00 g/mol = 0.150 g NaOH
The two answers differ by a factor of 1000. Writing “mL” next to the 25.0 would have flagged it right away.
Now the rounding trap. Suppose a gas problem gives P = 0.985 atm, V = 2.50 L and T = 298 K, and asks for the mass of N2 gas. Using n = PV/RT with R = 0.08206 L·atm/(mol·K), you get n = 0.10070 mol, and 0.10070 mol × 28.02 g/mol = 2.82 g. But if you round RT to 24 halfway through, n becomes 0.1026 mol and the mass becomes 2.87 g. On a multiple-choice exam, 2.82 and 2.87 can both be answer choices.
Don’t make this mistake: rounding each step to “clean” numbers. Keep at least one or two extra digits in your calculator until the final answer, then round once.
Getting Significant Figures Wrong at the End
Sig fig errors usually come from using the wrong rule for the operation. Addition and subtraction go by decimal places. Multiplication and division go by the number of significant figures. OpenStax lays out both rules for significant figures in calculations clearly if you need a refresher.
The fix is to decide your sig figs once, at the end, by looking back at the given data. On many multiple-choice exams, sig figs won’t separate the answer choices. On free-response exams, they often cost a point or two each time. Know which kind of exam you’re taking.
Calculator Entry Errors
Your calculator does exactly what you type, not what you mean. Two errors cause most of the damage.
The first is scientific notation. To enter 6.022 × 1023, use the EE or EXP key: 6.022 EE 23. A lot of students type 6.022 × 10 EE 23 instead, which gives 6.022 × 1024. That’s ten times too big.
The second is missing parentheses in the denominator. For the gas problem above, typing 2.4625 ÷ 0.08206 × 298 gives about 8,943. That’s nonsense. The calculator divided by R and then multiplied by T. You need 2.4625 ÷ (0.08206 × 298), which gives 0.1007.
The fix: put parentheses around every denominator that has more than one number, and do a quick estimate in your head before you trust the screen. It also helps to practice on the calculator you’ll actually use on test day. If you’re shopping for one, this guide to the best calculator for chemistry covers which keys matter and what instructors usually allow.
Misreading “NOT” and “EXCEPT” Questions
The symptom is choosing an answer that’s a true statement on a question that asked for the false one. Under time pressure, your brain reads what it expects to see.
The fix is physical. When you see NOT, EXCEPT, LEAST or FALSE in a question, circle it or underline it before you read the choices. Then, for each answer choice, mark it T or F. On a NOT question, you’re hunting for the single F.
Using the Wrong Mole Ratio
This shows up as an answer that’s off by a clean factor like 2, 3 or 1/2. That’s a giveaway that you flipped a ratio or skipped it.
Take 2 H2 + O2 → 2 H2O. The ratio of O2 to H2O is 1 to 2, so 1.0 mol of O2 makes 2.0 mol of water. Students who grab the coefficients in the wrong order get 0.50 mol instead.
The fix: write the mole ratio as a fraction with units, like (2 mol H2O / 1 mol O2). The unit you’re converting from goes on the bottom so it cancels. Also make sure the equation is balanced before you use any coefficients. If balancing still feels like guesswork, our article on how to balance chemical equations gives you an order that works every time.
Sign and Unit Errors in Thermochemistry
Thermo has three classic traps. First, when you reverse a reaction in a Hess’s law problem, the sign of ΔH flips. Second, in calorimetry, heat lost by one thing equals heat gained by the other, so q for the system and q for the surroundings have opposite signs. Third, ΔH is usually in kJ while ΔS is usually in J/K.
That third one is sneaky. Say ΔH = −92.2 kJ, ΔS = −198.7 J/K and T = 298 K. If you plug those straight into ΔG = ΔH − TΔS, you get a huge positive number and call the reaction nonspontaneous. Convert ΔS to −0.1987 kJ/K first and you get ΔG ≈ −33.0 kJ, which is spontaneous at that temperature. Same numbers, opposite conclusion.
The fix: before you calculate, write down the unit of every quantity and convert so they match.
Getting Stuck on One Hard Question
The warning sign is looking up and realizing you’ve spent eight minutes on one problem while easy questions sit untouched at the end. On most exams, a hard question and an easy one are worth the same.
The fix is a time budget. Divide the time limit by the number of questions to get your average per question. If a problem passes about twice that, mark your best guess, flag it and move on. Come back if time allows. Timed practice is the only way to make this feel natural. If you’re heading into a standardized final, set a timer and take our Gen Chem 1 ACS-style practice exam to practice pacing. Every question has a video solution, so you can see where a slip happened.
A Pre-Submit Checklist for Chemistry Exam Mistakes
If you have even three minutes left, use them on this list instead of staring at the clock.
- Did every calculation answer carry units, and do they match what was asked?
- Did I convert mL to L, grams to moles, and J to kJ where needed?
- Did I re-read every question with NOT, EXCEPT or LEAST in it?
- Is each answer a sensible size? (A gram-sized sample shouldn’t produce kilograms.)
- Did I answer every question, even with a guess, if there’s no penalty for wrong answers?
Stop Bleeding Points You Already Earned
Most chemistry exam mistakes aren’t about what you know. They’re about units, rounding, keystrokes, careless reading and pacing. Those are habits, and habits can be trained.
Pick the two mistakes from this article that show up most on your old exams. Practice the fix for each one on your next homework set, out loud if you have to. By test day, it should feel automatic.