Chem questions, mass?

Question

Determine the mass of aluminum that has the same number of atoms as there are in a) 6.29 mg Ag and b) 6.29 mg Au

Answer 1

Lets find out how many atoms there are in (a).

.00629gAg x (1molAg/107.9gAg) x (6.02x10^23atomAg/1molAg) = 3.51x10^19 atoms Ag.

Dont stop now!

.00629gAu x (1moAu/197.0gAu) x (6.02x10^23atomAu/1moAu) = 1.92x10^19 atoms Au

Now that we have counts on the atoms of both silver and gold, lets find out the mass of the same number of atoms in Aluminum.

3.51x10^19 atoms x (1mol/6.02x10^23) x (26.98gAl/1molAl) = 1.57mg Al

1.92x10^19 atoms x (1mol/6.02x10^23) x (26.98gAl/1molAl) = 0.860mg Al

Answer 2

You can save a lot of work if you notice they don't ask you to give them the number of atoms, just the mass.

For mass of Al with same number of atoms as 6.29mg of silver:

First, get in proper units: 6.29 mg = 0.00629g

[0.00629g/107.9g/mol] x 26.98g/mol = 0.001573g (1.573mg)

Same but with gold:

[0.00629g/196.97g/mol] x 26.98g/mol = 0.0008615g

(0.8615mg)