### Review: Molar Mass and MolesSolutions

You will use Al2(C7H6O2)3, aluminum benzoate, in this assignment. Please adhere to all rules for significant figures and put the appropriate units on each value.

(1) In a list, identify the elements in this compound by symbol and by name. How many of each are present? Include the atomic weight for each element.

 Name Symbol Number AtomicWeight aluminum Al 2 26.982 u carbon C 21 12.011 u hydrogen H 18 1.0079 u oxygen O 6 15.9994 u

(2) Calculate the molecular weight of this substance.

(2) (26.982 u) + (21) (12.011 u) + (18) (1.0079 u) + (6) (15.9994 u)

53.964 u + 252.231 u + 18.1422 u + 95.9964 = 420.334 u

Rounded off to three places in the decimal portion, following the significant figures rules for addition.

Depending on the source material, your answer may vary slightly from mine. Try this Internet molecular weight calculator to see this in action.

(3) Calculate the molar mass of this substance. (An older term for molar mass is gram-molecular weight.)

The molar mass of a substance is its molecular weight (units of amu or u) expressed in grams. The molar mass of aluminum benzoate is 420.334 g/mol.

(4) Define the chemical term 'mole.' What is the most common abbreviation for the term 'mole?'

Definition: a mole contains Avogadro Number of chemical entities. (Sometimes the value for Avogadro's Number is used.)

Not part of the definition, but usually accompanying the definition is a mention of some examples of chemical entities. Often mentioned are: atoms, molecules, ions, electrons.

The most common abbreviation for 'mole' is 'mol.'

(5) Calculate the grams present in (a) 1.00 mol of aluminum benzoate, (b) 0.00500 mole and (c) 1.84 x 10¯5 mole.

(1.00 mol) (420.334 g/mol) = 420. g (to three sig figs)

(0.00500 mol) (420.334 g/mol) = 2.10 g

(1.84 x 10¯5 mol) (420.334 g/mol) = 0.00773 g

(6) Calculate the moles pesent in (a) 253.0 grams of aluminum benzoate and (b) 2.00 grams

First one shows just the division, second one starts from a ratio and proportion.

 253.0 g –––––––––––– =  0.6019 mol 420.334 g/mol

 2.00 g 420.334 g ––––– = ––––––––– which leads to this: (2.00 g) (1 mol) = (420.334 g) (x) and thus to: x = 0.00476 mol x 1 mol

(7) How many molecules are present in 1.00 mole? What is the value of Avogadro’s Number (include the unit)?

1.00 mol of molecules contains 6.022 x 1023 molecules.

Avogadro's Number is 6.022 x 1023 mol¯1

(8) How many molecules of aluminum benzoate are present in (a) 0.850 mole and in (b) 4.380 grams?

(0.850 mol) (6.022 x 1023 mol¯1) = 5.12 x 1023 molecules

4.380 g / 420.334 g/mol) (6.022 x 1023 mol¯1) = 6.275 x 1021

(9) 4.550 x 1022 molecules of aluminum benzoate are present. How many (a) moles and (b) grams is this?

 4.550 x 1022 ––––––––––––––– =  0.07556 mol 6.022 x 1023 mol¯1

I reported a final answer rounded off to 4 sig figs just above, but I used more digits in the mole value in the calculation just below.

(0.0755563 mol) (420.334 g/mol) = 31.76 g

(10) What is the average mass in grams of one molecule of aluminum benzoate?

 420.334 g mol¯1 ––––––––––––––– =  6.980 x 10¯22 g 6.022 x 1023 mol¯1