1 mol 1 mol
Ex: How many grams of rubidium phosphide are there in 4.37x10
18 atoms of the compound?
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 Start the equation for the conversion by writing the known (given) with the unit and label (formula) Identify the conversion ratio to use 4.37x10
18 atoms Rb 3 P 1mol Rb 3 P 287.5g Rb 3 P 6.02x10
23 atoms Rb 3 P 1mol Rb 3 P = 2.1x10
-3 g Rb 3 P 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 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 Step 3 step 3a 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 Step 4 Na 2 Cr 2 O 7