Mary Beth J. answered • 06/29/22
High School & College Chemistry Tutor
Hi! These are both conversion questions. All you need to solve them is your periodic table and a calculator. The relationships you will be using are:
1 mole = 6.02x1023 representative particles (molecules, atoms, etc)
1 mole = molar mass of an element/compound (g/mol)
1. How many MOLECULES of sulfur dichloride are present in 8.04 grams of this compound? _____ molecules.
You always start with the number you are given: 8.04 g of SCl2 (sulfur dichloride - 1 sulfur and 2 chloride)
Then you identify what you need: the number of molecules
Since we have no relationship directly from grams to molecules, we will have to convert to moles first and then to molecules. To convert grams into moles, we need the molar mass for SCl2. This is where the periodic table is because atomic mass is numerically equivalent to molar mass. The molar mass therefore for Sulfur is 32.065 (listed on the element box of the P.T.) and the molar mass of Chlorine is 35.453. The molar mass of sulfur dichloride is then = 32.065 + (35.453 x 2) = 102.971 g/mol
You can then set up a set of conversion tables like this. You multiply all things on the top of the fractions and divide by all things on the bottom.
8.04 g SCl2 x 1 mole SCl2 =. 0.078 moles of SCl2
1 102.971 g
0.078 moles SCl2 x 6.02x1023 molecules = 4.70x1022 molecules of sulfur dichloride
1 1 mole SCl2
2. How many GRAMS of sulfur dichloride are present in 9.25×1022 molecules of this compound? _____ grams.
Here we are using the same relationships as before, but are going in the opposite direction: starting with molecules, converting to moles and then finally to grams. You could do this in one big conversion table instead of breaking it into two distinct steps. *Remember that units on the numerator and the denominator simplify to 1 and therefore can be canceled out.*
9.5x1022 molecules SCl2 x 1 mole SCl2 x 102.971 g SCl2 = 16 grams of sulfur dichloride
1 6.02x1023 molecules 1 mole SCl2
