A Final-Answer Checklist for Chemistry Numericals
Many students lose marks in chemistry numericals not because they do not know the method, but because of small slips at the very end: a missing unit, a wrong power of ten, an answer that is impossible. This article gives a short checklist you can run in 30 seconds before you write the final answer. It works for mole concept, solutions, gas laws, thermodynamics and equilibrium problems in Class 11 and 12.
The six-step checklist
Each step takes a few seconds. Do them in this order, and tick them off on the rough page.
Step 1: Units
Write the unit after every number in your working, not only in the answer. If the unit of the result is not the unit the question asks for, something is wrong. Common conversions: mL to L (divide by 1000), g to kg (divide by 1000), °C to K (add 273), and kJ to J (multiply by 1000). In the gas law, R = 0.0821 L atm K−1 mol−1 goes with litres and atmospheres, while R = 8.314 J K−1 mol−1 goes with joules.
Step 2: Size
Ask: is this answer reasonable? A few anchors help. A molarity of a normal laboratory solution is usually between 0.001 and 12 mol/L. A pH is between 0 and 14 for ordinary solutions. A percentage cannot be more than 100. A mole fraction is between 0 and 1. The mass of product cannot be more than the total mass of the reactants. If you get a mole fraction of 1.4 or a yield of 130%, stop and find the slip.
Step 3: Sign
In thermodynamics, the sign carries meaning. An exothermic reaction has a negative ΔH. A gas expanding against a surrounding pressure does work on the surroundings, so check your sign convention (ΔU = q + w in the NCERT convention, where w is work done on the system). A spontaneous reaction has a negative ΔG. For a cell, a positive E°cell means a spontaneous reaction.
Step 4: Significant figures
Give the answer to the same number of significant figures as the least precise data value, unless your teacher asks for a fixed number of decimals. Do not round in the middle of the calculation. Round only the final answer.
Step 5: Second route
Where possible, check the answer by a different method. Put the answer back in the original equation and see whether it fits. This is the most powerful step and it is described in the examples below.
Step 6: Re-read the question
Many students answer a different question from the one asked. Did the question ask for molarity or molality? Mass of the product or moles? The value of ΔH per mole or for the whole mass? Underline the word that tells what is wanted.
Worked example 1: a mole concept slip
How many grams of NaOH (molar mass 40 g/mol) are needed to make 250 mL of a 0.20 M solution?
Wrong working: moles = 0.20 × 250 = 50, mass = 50 × 40 = 2000 g. Size check: 2 kg of NaOH to make a quarter of a litre of solution is clearly impossible, so the unit of volume was not converted.
Correct working: V = 250 / 1000 = 0.250 L. Moles = 0.20 × 0.250 = 0.050 mol. Mass = 0.050 × 40 = 2.0 g.
Second route: 1 L of 0.20 M needs 0.20 × 40 = 8.0 g. A quarter of a litre needs a quarter of that: 8.0 / 4 = 2.0 g. Both routes agree.
Worked example 2: a gas law numerical
Find the pressure of 0.50 mol of an ideal gas in a 10.0 L vessel at 27 °C.
T = 27 + 273 = 300 K. P = nRT / V = (0.50 × 0.0821 × 300) / 10.0 = 12.315 / 10.0 = 1.23 atm.
Size check: At 273 K and 1 atm, 1 mol occupies about 22.4 L, so 0.50 mol would occupy about 11.2 L. Here the gas is in 10.0 L, a little less, and the temperature is a little higher. A pressure a little above 1 atm is sensible. If you had forgotten to convert to kelvin (using 27 instead of 300), you would have got P = 0.11 atm, which fails the size check.
Worked example 3: pH and the second route
Find the pH of 0.0010 M HCl, a strong acid.
[H+] = 1.0 × 10−3 mol/L, so pH = −log(1.0 × 10−3) = 3.00.
Second route: pOH = 14 − 3.00 = 11.00, so [OH−] = 10−11 and [H+][OH−] = 10−3 × 10−11 = 10−14 at 25 °C. The check closes. Also, a strong acid at 10−3 M should have an acidic pH, below 7. The answer 3.00 passes. Significant figures: the concentration has two significant figures, so the pH is written with two decimal places (3.00).
Worked example 4: a percentage that cannot be right
A reaction should give 8.0 g of product in theory. The student collects 6.4 g. Find the percentage yield.
Yield = (6.4 / 8.0) × 100 = 80%.
Size check: the actual mass is smaller than the theoretical mass, so the answer must be below 100%. If you had divided the other way, 8.0 / 6.4 × 100 = 125%, and the size check would catch it at once.
Mistakes the checklist catches
- Writing the final number without any unit, or with the wrong one.
- Using Celsius instead of kelvin in gas laws and in rate expressions.
- Using mL where litres are needed in molarity.
- Rounding early and then losing the last digit of the answer.
- Answering for the whole sample when the question asked for one mole, or the opposite.
- Copying the number from the calculator display without checking the power of ten.
Using the checklist in the exam hall
| Step | Time needed | Typical error it catches |
|---|---|---|
| Units | 5 seconds | mL not converted, wrong R |
| Size | 5 seconds | Mole fraction above 1, yield above 100% |
| Sign | 5 seconds | ΔH, ΔG, E° with wrong sign |
| Significant figures | 5 seconds | Too many or too few digits |
| Second route | 10 to 20 seconds | Arithmetic slips |
| Re-read question | 5 seconds | Answering a different quantity |
Practise the checklist on every homework numerical for two weeks, until it becomes a habit. Do not plan to start it only in the exam. How marks are given for units and for method differs by board and by teacher, so follow the marking pattern in your board’s latest sample paper.
You can use the Scientific Calculator on the home page to check each calculation in your second-route step.
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