The mass of a beaker containing a sample of sodium dichromate was determined to be 68.9 g. After transferring a sample it weighed 56.424 g.Determine the mass (g) of sodium dichromate transferred to the correct number of decimal places
68.9
-56.424
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=12.476 but you may not use that many places. In subtraction, you may use as many places to the right of the decimal as the LEAST precise which means the 24 of the .424 number wont count and the answer is 12.5 g.
You can read about significant figures for multiplication/ division and for addition/subtraction here. It's a good site.
http://www.chemteam.info/SigFigs/SigFigs.html