Download Determining Molar Masses: HCl, BeSO4, and GaCO3 and more Slides Chemistry in PDF only on Docsity!
Finding the molar mass:
Examples : a) HCl
H 1 1. 1
g mol g mol
= × =
Cl 1 35. 1
g mol g mol
= × =
1.01 g + 35.45 g =36.46 g
So the molar mass of HCl (hydrogen chloride) is 36.46 grams per mole ( 36. g mol ).
b) BeSO (^4)
Be 1 9. 1
g mol g mol
= × =
S 1 32.
g mol g mol
= × =
O 4 64
g mol g mol
= × =.
9.01 g + 32.06 + 64.00 g =105.07 g
So the molar mass of BeSO 4 (beryllium sulfate) is 105.074 grams per mole (105. g mol ).
This type of problem can be done using less space by combining the above steps into one. Instead of finding the mass of each individual element in the compound and then adding them together, do both as you go as follows:
c) Ga2(CO 3 ) (^3)
69.72 12.01 16. 2 Ga 3 C 9 O 319. 1 Ga 1 C 1 O
g g g mol mol mol g mol mol mol
× + × + × =
So the molar mass of Ga2(CO (^) 3) 3 (gallium carbonate) is 319.74 g mol.
Your turn. Using the same steps as above, find the molar mass of each of the following compounds. Don’t let the large molecules fool you; the same steps are used in each calculation. You might also want to try naming the compounds to help with nomenclature (except numbers 6, 9, 13, 14).
1) H 2O
- HgO
- Hg 2 O
- I (^2)
- I
- CH (^) 3OH
- Ca(NO 3 ) (^2)
- Al(OH) (^3)
- C 12 H 22 O (^11)
- Ba 3 (PO (^) 4) (^2)
- Sr(MnO 4 ) (^2)
- (NH (^) 4) (^) 2Cr 2 O (^7)
- PtCl2(NH 3 ) (^2)
- C 23 H 40 N (^) 7O (^) 17P 3 S
Dealing with moles:
Avogadro’s number is the key to this type of problem. Everyone knows what is meant when someone says they have a dozen somethings. It is automatic that a dozen means there are 12 of whatever somethings are being talked about. Avogadro’s number is the same thing except a little bigger and is used to describe the number of particles in one mole of substance. If I told you that I had a dozen pencils and asked you to tell me how many pencils I have, this is what you would do to find out:
pencils dozen × = pencils dozen
You can also find how many dozen pencils you have if you have 60 pencils:
1 60 ls 5 ls
× =
dozen dozen 1
penci 2 penci
Just as 1 dozen pencils contains 12 pencils, 1 mole of particles contains 6.022x
23 particles. These particles can be
anything, but in this class, they will always be atoms or molecules. A problem asking how many atoms there are in a certain
number of moles will be set up EXACTLY the same way as the proceeding example of 3 dozen pencils.
Examples:
a) How many atoms are there in 5.62 moles of Potassium?
23 6.02 (^1024) 5.62 3.38 10 1
×
× = ×
atoms mol atoms mol
b) How many moles are there in 1.16x
22 molecules of Br 2?
22 23
× × =
×
molecules molecules
- How many moles are there in 5.87x
24 molecules of NaCl
mol mol
Again it is your turn. As a hint for this review, as well as your tests and quizzes, EVERY time you see the phrase “how many
atoms…” or “how many molecules…” in a question, you WILL use Avogadro’s number!
How many moles are there in 1 mol of moles?
How many atoms are there in 7.42 moles of tin?
How many molecules are there in 3.14 moles of water?
How many ATOMS are there in 8.22 moles of H 2 molecules?
How many moles are there in 9.46x 18 molecules of AgNO 3?
Now for a little twist. These will be basically the same questions with one added step. In this case, you will be asked how
many atoms or moles are represented by one PART of a molecule.
Examples:
a) How many sodium atoms are there in 3.75 moles of Na3PO 4?
The first step is to determine the number of sodium moles in one mole of Na (^) 3PO 4. Look at the subscript to find out. There are 3 sodium moles in each mole of the compound. So: 23 3 6.02 (^1024) 3.75 6.77 10 1 1
×
×
Na Na
atoms 3 4 ×^ ×^ = atoms 3 4
N
Na a PO Na PO Na
mol mol
mole
mole
b) How many sulfur atoms are there in .512 moles of SO 2?
Once again, how many moles of sulfur are there in each mole of SO 2? Looking at the subscript, we see that there is one mole of sulfur for each mole of SO (^) 2. So: 23 1 6.02 (^1023) .512 3.08 10 1 1
×
×
S
S
atom 2 ×^ ×^ = atom 2
S
S
S
O
O
S
mol mol
mol
mol
c) If there are 7.84x
24 atoms of oxygen in a sample of hydrogen peroxide (H (^) 2O (^) 2), then how many moles of hydrogen peroxide are present? This is the last question, only in reverse. First find the number of moles of oxygen atoms in each mole of H (^) 2O (^) 2, which from the subscript, is 2. So:
24 23
× × × =
2 2 2 2
H O
H O
mol m lo ×
O
O
O
O
atoms atom
mol
s mol
NOTE: The number of moles of one substance over the number of moles of another substance is called the mole to mole ratio. In the preceding example, the mole to mole ratio is 1:2, meaning there is one mole of H (^) 2O 2 for every two moles of O. The mole to mole ratio is used in the majority (but not all) of the chemical calculations you will do in this class, so learn it well!
