A 400g sample is found to be 27% aluminum. What is the mass percent of A1 in the sample?

34.59%

Explanation:
In order to calculate the mass percent of aluminum in that ionic compound, we're going to need two things:
1: The formula mass of the entire compound

2: The atomic mass of aluminum

The formula mass of aluminum hydroxide is
78.00
g
m
o
l
because:
26.98
g
m
o
l
+
3
(
16.00
g
mol
)
+
3
(
1.0079
g
mol
)

=
78.00
g
m
o
l

We already know the atomic mass of aluminum since that's already given.
Now, we have to use the following equation:
https://wilsonsch3u-01-2012.wikispaces.com/Unit+4+Moles?responseToken=6fbe1bc52003b1c7d0a470e50127b3a2
https://wilsonsch3u-01-2012.wikispaces.com/Unit+4+Moles?responseToken=6fbe1bc52003b1c7d0a470e50127b3a2
The numerator represents the mass of the desired atom, which is Al in our case, and the denominator represents the mass of the entire compound. You just divide the two and multiply by 100 to obtain the percent composition:
Mass of Al
Molar Mass of Al(OH)
3
×
100
%

26.98 g/mol
78.00g/mol
×
100
%
=
34.59
%