Question #383ec

1 Answer
Apr 8, 2017

Answer:

#"43.3 g mol"^(-1)#

Explanation:

For starters, you can use the mass of the compound and the mass of oxygen to determine how many grams of element #"A"# you have in your sample

#"mass of compound = mass of A + mass of O"#

In your case, you will have

#"mass of A" = "0.359 g" - "0.128 g" = "0.231 g"#

Now, the problem wants you to determine the molar mass of substance #"A"#, which implies that you must figure out the mass of exactly #1# mole of #"A"#.

Use the molar mass of oxygen to calculate the number of moles of oxygen present in the sample

#0.128 color(red)(cancel(color(black)("g"))) * "1 mole O"/(16.0 color(red)(cancel(color(black)("g")))) = "0.00800 moles O"#

According to its chemical formula, the oxide contains #2# moles of #"A"# for every #3# moles of oxygen. This means that the sample must contain

#0.00800 color(red)(cancel(color(black)("moles O"))) * "2 moles A"/(3color(red)(cancel(color(black)("moles O")))) = "0.00533 moles A"#

Since you know that #0.00533# moles of #"A"# have a mass of #"0.231 g"#, you can say that #1# mole of #"A"# will have a mass of

#1 color(red)(cancel(color(black)("mole A"))) * "0.231 g"/(0.005333color(red)(cancel(color(black)("moles A")))) = "43.3 g"#

Therefore, you can say that element #"A"# has a molar mass of

#color(darkgreen)(ul(color(black)("molar mass A = 43.3 g mol"^(-1))))#

The answer is rounded to three sig figs.