How many grams of SF6 should be added to a sample of 486 g SF6 to obtain 5 moles of the compound?

Question

How many grams of SF6 should be added to a sample of 486 g SF6 to obtain 5 moles of the compound?
244 g
730 g
114 g
33 g
none of the above, as 486 g represents more than 5 moles


What I understand is 5 Mol of SF6 x (146.07 g SF6/ 1 mol SF6 ) = 730.2 g SF6

Now do you divide that? and get

244.2g

or is the answer 730.2g?

Answer 1

Neither.
It seems to me that your trouble is in understanding the importance of "significant figures".
Yes, from the periodic table or some literatures, you may find the molar mass of SF6 accurate to many digits. Here you took four digits. Fine.
However, for a weight of SF6 you are given 486g, which could be as high as 486.49999 g, or as low as 485.5 g. Now you subtract this weight from the total weight of 730.2g, you know the number is between 243.70001g to 244.7g. Hence the last digit of your 244.2g is completely meaningless. So, please review the rules as how to take the appropriate significant figures.
The answer is 244g

Answer 2

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