53.3% + 6.7% = 60%, 100% - 60% = 40%. 40% of glucose is made of carbon. Since there are only three types of atoms in glucose, and the amount of hydrogen and oxygen is already given, this means that whatever percentage is left (40%) has to be carbon.
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Volume in mL :
1.68 L * 1000 => 1680 mL
Density = 0.921 g/mL
Therefore:
Mass = density * Volume
Mass = 0.921 * 1680
Mass = 1547.28 g