Question #68fe6

1 Answer
Jul 4, 2016


#"0.092 g"#


The problem wants you to use the number of moles, #n#, present in a sample of a given compound and the molar mass, #M#, of said compound to determine the mass of the sample, #m#.

The idea here is that a compound's molar mass can be used as a conversion factor between number of moles and mass expressed in grams.

That is the case because the molar mass essentially tells you the mass of one mole of a given compound. In your case, the compound is said to have a molar mass of

#M = "44 g/mol"#

This tells you that every mole of said compound has a mass of #"44 g"#.

Since you know that the sample contains #0.0021# moles, you can say that its mass will be equal to

#0.0021 color(red)(cancel(color(black)("moles"))) * overbrace("44 g"/(1color(red)(cancel(color(black)("mole")))))^(color(purple)("= molar mass")) = color(green)(|bar(ul(color(white)(a/a)color(black)("0.092 g")color(white)(a/a)|))) -># rounded to two sig figs.

This can also be calculated by using the equation

#color(blue)(|bar(ul(color(white)(a/a)m = n * M_Mcolor(white)(a/a)|)))#

Since the mass of the sample is equal to the number of moles it contains multiplied by the molar mass of the compound, you will once again have

#m = 0.0021 color(red)(cancel(color(black)("moles"))) * "44 g"/color(red)(cancel(color(black)("mol"))) = color(green)(|bar(ul(color(white)(a/a)color(black)("0.092 g")color(white)(a/a)|)))#

The answer must be rounded to two sig figs because that's how many sig figs you have for the number of moles present in the sample.