Final answer:
The least precise digit in the measured mass of iron (2.756 g) is the third digit, 5. This reflects the uncertainty of ±1 in the least significant figure of the measurement, which depends on the precision of the measuring device.
Step-by-step explanation:
In the measurement of a sample of iron with a mass of 2.756 g, the digit with the least certainty is the third digit, which is 5. In the context of significant figures, each digit in a measurement provides some level of certainty, with the rightmost digit always being the least certain. The measured value of 2.756 g suggests that the iron's mass is between 2.755 g and 2.757 g. This uncertainty is due to the precision of the measuring device and the ability of the user. In general, the last digit in a measurement is understood to be an estimate and is thus considered to have an uncertainty of ±1 in the least significant figure, which is typically the place value of the final digit provided by the measuring instrument.
For example, if a balance reports the mass as 1573 g, the actual mass could range from 1572.5 g to 1573.4 g, representing an uncertainty of ±1 g. When adding or subtracting measurements, the final answer must reflect the least precise measurement involved. This means rounding the answer to maintain the same level of precision, signified by the least certain digit in the original measurements.