Molar conversions:
23
1 mol 1 mol 1 mol
Ex: How many grams of rubidium phosphide are there in 4.37x10
18 atoms of the compound?
1.
Identify the known/given (in red) and the unknown (in blue) from the problem. Determine the formula of the compound in the problem, if not already given.
Known: 4.37x10
18 atoms Rb
3
P Unknown: grams Rb
3
P
2.
Start the equation for the conversion by writing the known (given) with the unit and label (formula)
3.
Identify the conversion ratio to use
4.37x10
18 atoms Rb
3
P 1mol Rb
3
P 287.5g Rb
3
P = 2.1x10
-3 g Rb
3
P
6.02x10
23 atoms Rb
3
P 1mol Rb
3
P
4.
Cancel like terms – do calculations OR identify the next conversion ratio to use and calculate answer
To determine Empirical Formulas:
Ex: A compound contains 17.6% sodium, 39.7% chromium, and 42.7% oxygen. What is the empirical formula?
1.
Change % to g (grams)
2.
Convert grams to moles, using molar mass, found on the periodic table
3.
Identify the smallest mole answer from step number 2 and divide all the answers by that smallest number a.
If the answer is not a whole number, you must multiply it by some number (2,3,4) to get a whole number
4.
Write the formula placing the number calculated from step 3 as a subscript
Step 1 Step 2 Step 3 step 3a Step 4
Na
2
Cr
2
O
7
17.6% Na = 17.6g Na 17.6g Na 1mol Na = 0.77 mol
23.0g Na
39.7% Cr = 39.7g Cr 39.7g Cr 1mol Cr = 0.76 mol
52.0g Cr
42.7% O = 42.7g O 42.7g O 1mol O = 2.67 mol
42.7g O
0.77 mol = 1 x2 =2
0.76
0.76 mol = 1 x2 =2
0.76
2.67 mol = 3.5 x2 =7
0.76