Question

The mass of a glass beaker is known to be 24.8 g. Approximately 5 mL of water are added, and the mass of the beaker and water is measured on an analytical balance to
be 30.625 g How many significant figures are there in the mass of the water?

Answer 1

Answer:

• Two signficant figures.

Explanation:

You need to tell how many significant figures there are in the mass of water, which is calculated as the difference of two measures: 30.625 - 24.8.

You must round the mass of water to the least number of places in the decimal portion found in the two numbers that participate in the subtraction.

The least number of places in the decimal part corresponds to the measure 24.8 g and it is 1 decimal.

So, your answer must be rounded to 1 decimal:

• Subtraction: 30.625 g - 24.8 g = 5.825 g

• Round to 1 decimal: 5.8 g

Some rules that you must follow to determine how many significant figures there are in a measure are:

•  Every non-zero digits in number are always significant.

•   Also every zero between two significant digits are significant.

•  Zeros in between the decimal point  and the first a non-zero decimal digit are not significant.

• In a whole number final zeros cannot be taken as significant (e.g. it is ambiguos the number of significant digits in measures as 3,300).

• Trailing zeros to the right of a decimal point are significant.

So, in the 5.8 you apply the first rule: non-zero digits are always significant. So, the two digits in 5.8 are significant.