The balance that can be used to measure a mass reading of 6.521 g will be a balance with ±0.001 g tolerance.
The tolerance of a balance is the number of decimal place values displayed on the balance and it is the acceptable measurement limitations (in grams) that a balance can tolerate.
Since no mass reading was given, let's make two assumptions about mass reading.
Mass reading Assumptions
1 6.521 g
2 4.23 g
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