Posted by Bryan on .
Classify each of these statements as always true, sometimes true, or never true.
7____AT____It is necessary to know the formula of a compound in order to calculate its percent composition.
8___ST_____If the percent by mass of carbon in methane, CH4, is 75%, then 100 grams of methane contain 25.0 grams of hydrogen.
9____AT____The formula for methane, CH4, is both a molecular and an empirical formula.
10____NT___The empirical formula for glucose, C6H23O6, is C2H4O2.
7. I may not understand this question very well but I can calculate the percent composition if I know the grams of A that combine with the grams of B as well as if I know the formula.
8. You need to rethink this. If CH4 is 75% C it must be 25% H and 100 g CH4 will contain 25g H. If it is sometimes true, when is it not true?
9. I go along.
10. Glucose is C6H12O6; therefore, the empirical formula is CH2O. Since the incorrect formula is given for glucose I don't know how to answer this.
Help on Completion.
The Percent by mass of an element in a compound is the number of grams of the element per _____g of the compound, multiplied by 100%.

Chemistry (Check) 
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The "Help on Completion." question is the same one I am struggling with! ahhh!