Chemistry: Accounting for Matter

Molar Mass

100

What is a mole? What is molar mass?

• The mole is the "counting unit" used by chemists to indicate the number of atoms, ions, molecules, or formula units present in a particular chemical sample. 

• The mole is similar to other counting units that you've used before....pair (2), dozen (12), and gross (144). 

• The molar mass of a compound tells you the mass of 1 mole of that substance. In other words, it tells you the number of grams per mole of a compound. So the units for molar mass are grams/mole.

200

What is the molar mass of this compound: NaBr

102.9 g/mol

300

(NH₄)₂CO₃

96.0 g/mol

400

H₃PO₄

97.994 g/mol

500

Determine the molar mass of Fe₂(SO₄)₃.

399.91 g/mol

Particles to Moles or Molecules

100

Two different substances each contain one mole. Which statement is always true?
A) They contain the same mass.
B) They contain the same number of particles.
C) They contain the same number of protons.
D) They contain the same volume.

B) They contain the same number of particles.

200

How many moles are there in 458 grams of Na₂SO₄?

3.22 moles Na₂SO₄

300

How many molecules are there in 122 moles of NO₂?

1.60 x 10²⁴ molecules

400

One mole of CaCl₂ contains how many chlorine atoms?
A) 6.02×10²³ atoms   

B) 1.20×10²⁴ atoms    

C) 3.01×10²³ atoms    

D) 2 atoms

B) 1.20×10²⁴ atoms

500

How many atoms are in 3.5 mol of Al₂(SO₄)₃ ?

2.11 x 10²⁴ particles of Al₂(SO₄)₃

Grams to Moles/Moles to grams

100

How can you convert grams to moles?

To convert grams to moles, start by multiplying the number of atoms by the atomic weight for each element in the compound. Then, add all of your answers together to find the molar mass of the compound. Finally, divide the number of grams of the compound by the molar mass of the compound to find the number of moles.

200

How many moles of KCl are in 27.5 g of KCl?

0.369 moles KCL

300

Determine the number of grams from 1.70 moles KMnO₄.

0.369 moles 

400

True or False: 

One mole of nitrogen gas (N₂) and one mole of oxygen gas (O₂) contain the same number of _______ but different _______.

particles; masses

500

How many moles are in 72 g of H₂O?

4 moles H₂O

Percent Composition

100

Calculate the percent composition of Na in NaHSO₄?

% Na = 22.99/120.06 x 100% = 19.15% 

200

Calculate % O in CaCO₃.

48/100 x 100%= 48%

300

Define percent composition

Percent composition is the percentage by mass of each element in a compound. 

Example: The percent composition of water is 20% hydrogen and 80% oxygen

400

The total percent composition of a compound always adds up to ______%

100%

500

Which compound has the highest percent of hydrogen by mass?
A) H₂O  

B) CH₄  

C) NH₃  

D) C₂H₆

D) C₂H₆

Empirical & Molecular

100

What is an empirical formula?

A formula that gives the simplest whole-number ratio of atoms in a compound.

200

Determine the empirical formula of the following: 

13.5 g Ca, 10.8 g O, and 0.675 g H

 CaO₂H_{2 ------>} Ca(OH)₂

300

Determine the empirical formula of the following:

40.0 g C, 6.7 g H, and 53.3 g O

CH₂O

400

What is a molecular formula?

• true formula

• tells us the actual number of the different elements in one molecule of a compound.

500

Determine molecular formula:

CH₂O - empirical formula

Molar Mass: 90 g/mol

C₃H₆O₃