Chemistry · Some Basic Concepts Of Chemistry · NEET
NCERT gives three rules based on the first digit you drop. Rule 1: if that digit is more than 5, add one to the digit kept (1.386 → 1.39). Rule 2: if it is less than 5, keep the digit the same (4.334 → 4.33). Rule 3: if it is exactly 5, leave the kept digit unchanged if it is even, but raise it by one if it is odd. So 6.25 → 6.2 (2 is even) and 6.35 → 6.4 (3 is odd).
Because the digit being dropped is exactly 5, so you use the even rule. The digit before the 5 is 2, which is already even, so it stays the same. That gives 6.2. This surprises many students because in school they were told 'always round 5 up'. NEET follows NCERT, so use the even rule for an exact 5.
It is the very first digit you cut off from the right, at the place where you stop. If you round 34.216 to three significant figures, you keep 34.2 and the first removed digit is 1. Only that one digit decides what happens. Do not add up the removed digits or look at digits further right.
Only when 5 is the exact first digit removed and nothing meaningful comes after it. If a non-zero digit follows the 5 (like 6.253 → drop 53), then the removed part is more than 5, so you simply round up to 6.3. The even rule is only for a clean, exact 5.
Counting significant figures tells you how many meaningful digits a number has. Rounding is the action you do after a calculation to cut the answer down to that allowed number of significant figures. You first decide how many figures to keep (from the least precise measurement), then you round off to reach it.
Round only the final answer. If you round in the middle of a long calculation, small errors build up and your final value can be wrong. Carry extra digits through the steps and round once at the end to the correct number of significant figures.
The numbers 17.0145 and 21.0235 were rounded to three figures after the decimal point. The resulting numbers, respectively, are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Keep the digit before the 5 the same if it is even, and raise it by one if it is odd. So 6.25 → 6.2 (2 is even) and 6.35 → 6.4 (3 is odd). This is the NCERT even rule and NEET follows it.
Keep 34.2 and look at the first removed digit, which is 1. Since 1 is less than 5, the kept digit stays the same. The answer is 34.2.
Leading zeros do not count, so the first three significant figures are 4, 5, 9. The next digit is 7, which is more than 5, so round up. The answer is 0.0460.
No. You first decide how many significant figures the answer is allowed to have, usually from the measurement with the fewest figures. Rounding only trims the extra digits to reach that number.
Chemistry numerical answers in molarity, mole and mass problems must be reported with the correct significant figures. Picking the wrong rounded value, especially with an exact 5, can make you choose the wrong option in the exam.
