The atomic weight of aluminum is 26.982 u, how many aluminum atoms are there in a 4.55g sample of aluminum?
The key to this problem is the correct interpretation of what atomic weight means.
Notice that aluminium is said to have an atomic mass of
The unified atomic mass unit is defined as
#"1 u" color(white)(x) = color(white)(x)"1 g/mol"#
This means that the mass of one mole of aluminium atoms will be
As you know, one mole of any element contains exactly
This means that if you know how many moles of aluminium you have in
#4.55 color(red)(cancel(color(black)("g"))) * "1 mole Al"/(26.982color(red)(cancel(color(black)("g")))) = "0.168631 moles Al"#
Therefore, you will have
#0.168631color(red)(cancel(color(black)("moles of Al"))) * (6.022 * 10^(23)"atoms of Al")/(1color(red)(cancel(color(black)("mole Al")))) = color(green)(1.02 * 10^(23)"atoms of Al")#
The answer is rounded to three sig figs, the number of isg figs you gave for the mass of the sample.