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Balancing Chemical equations

Moles

Objectives

• Balance chemical equations

• Calculate the molar mass of different compounds

• Calculate the mass of given number of moles and number of moles of given mass

Balancing chemical equations

A balanced chemical equation represents a chemical reaction using the formulae of the reactants and products. It shows the number of units of each substance involved.

The law of conservation of mass states that no atoms are lost or made during a chemical reaction, so the total mass of the products is equal to the total mass of the reactants.

Balancing chemical equations

This means that chemical reactions can be represented by symbol equations. A balanced symbol equation has the same number of atoms of each element on both sides of the arrow.

To balance an equation, add numbers to the left of one or more formulae. 

Multiple Choice

A balanced chemical equation represents a chemical reaction using the formulae of

• Reactants only

• Products only

• Reactants and products

Multiple Choice

The law of conservation of mass states that

• atoms are lost or made during a chemical reaction

• no atoms are lost or made during a chemical reaction

Multiple Choice

A balanced symbol equation has the same number of atoms of each element on both sides of the arrow.

• True

• False

Multiple Choice

Which numbers correctly fill the gaps to balance this equation?

__Na + Cl₂ → __NaCl

• 2, 4

• 4, 2

• 2, 2

Multiple Choice

Which numbers correctly fill the gaps to balance this equation?

C₃H₈ + __O₂ → __CO₂ + __H₂O

• 3, 3, 2

• 10, 3, 8

• 5, 3, 4

Moles

Chemists measure the amount of a substance in a unit called ‘the mole’.

It allows chemists to make predictions about the masses of different substances that are involved in reactions.

Molar mass

Recall that the molar mass is the mass, in grams, of one mole of a substance.

To calculate the molar mass of a compound:

Multiply the mass number of the element (from the periodic table) by the number of atoms of the same element in the compound.

Multiple Choice

What is the molar mass of O₂?

• 16

• 8

• 32

Multiple Choice

Calculate the molar mass of Ba(C₂H₃O₂)₂

• 255.3 g/mol

• 237.3 g/mol

• 228.3 g/mol

• 196.3 g/mol

Multiple Choice

Calculate the molar mass of CO₂.

• 14 g/mol

• 28 g/mol

• 36 g/mol

• 44 g/mol

Multiple Choice

What is the molar mass of NaOH?

• 40 g/mol
• 38.989 g/mol
• 23.998 g/mol
• 57.004 g/mol

Multiple Choice

What is the molar mass of sodium?

• 11

• 23

• 45.98

• 3

Multiple Choice

What is the molar mass of sodium?

• 11
• 23
• 45.98
• 3

Moles

Remember:

$\t$ $n=\frac{m}{Mm}$  
OR
 $m=n\times Mm$  

Multiple Choice

Which of the following would have the highest number of moles?

• 4g CO₂
• 15g H₂O
• 22g NaCl
• 100g AgCl

Multiple Choice

Vitamin C, also known as ascorbic acid, is water soluble and cannot be produced by the human body. Each day, a person's diet should include a source of Vitamin C, such as orange juice. Ascorbic acid has a molecular formula of C₆H₈O₆ and a molar mass of 176 grams per mole.

Determine the number of moles of vitamin C in an orange that contains 0.171 grams of vitamin C.

• 30.1 moles

• 1030 moles

• .000971 moles

• .0001 mole