Test #6- Stoichiometry Review

Given the balanced equation representing a reaction:

CaO(s) + CO₂(g) --> CaCO₃(s) + heat

What is the total mass of CaO(s) that reacts completely with 88 grams of CO₂(g) to produce 200 grams of CaCO₃(s)?

• 56 g

• 88 g

• 112 g

• 288 g

Answer explanation

Recall law of mass conservation

Mass on both sides of a chemical reaction must be equal!

CaO + CO₂ --> CaCO₃

x + 88 g --> 200 g

x = 112 g

Tags

NGSS.HS-PS1-7

The mass of a mole of a substance is equal to

• the atomic number in grams
• the mass of the most common isotope in grams
• the gram-formula mass

• the mass of 22.4 L of any substance

Answer explanation

The mass of 1 mole of any substance is known as the gram-formula mass.

Tags

NGSS.HS-PS1-7

Which equation is correctly balanced?

• H₂ + O₂ --> H₂O

• N₂ + H₂ --> NH₃

• 2NaCl --> Na + Cl₂

• 2KCl --> 2K + Cl₂

Answer explanation

Recall law of mass conservation

Mass and number of atoms on each side of a chemical reaction must be equal!

2KCl --> 2K + Cl₂

Left side: Right Side:

K-2 K-2

Cl-2 Cl-2

Both sides are equal. This reaction is balanced.

Tags

NGSS.HS-PS1-7

What is the empirical formula for the compound C₆H₁₂O₆?

• CH₂O

• C₂H₄O₂

• C₃H₆O₃

• C₆H₁₂O₆

Answer explanation

Recall the difference between empirical and molecular formulas.

Empirical formula: The simplified/reduced version of the molecular formula.

Molecular formula: The formula of a covalent compound that shows the actual number of atoms in the compound.

Given:

Molecular formula of glucose = C₆H₁₂O₆

Divide all subscripts by 6.

Empirical formula of glucose = CH₂O

What is the total number of moles in an 80-gram sample of C₂H₅Cl (gram-formula mass = 64 g/mol) ?

• 0.806 mol
• 1.24 mol
• 2.48 mol
• 5.16 mol

Answer explanation

Use the Mole Calculation equation on the back of the reference table.

Mole = 80 grams/ 64 g/mol

= 1.24 moles

Tags

NGSS.HS-PS1-7

Consider the following unbalanced equation:

_Fe + _O₂ --> _Fe₂O₃

When the equation is completely balanced using smallest whole numbers, the coefficient of Fe is

• 1
• 2
• 3

• 4

Answer explanation

Given:

Fe + O₂ --> Fe₂O₃

Balance the reaction

Left Side: Right Side

Fe - 1 Fe -2

O - 2 O -3

2Fe + O₂ --> Fe₂O₃

Left Side: Right Side:

Fe - 2 Fe- 2

O - 2 O - 3

Balance O by using the least common multiple of 2 and 3 (6).

2Fe + 3O₂ --> 2Fe₂O₃

Left Side: Right Side:

Fe - 2 Fe - 4

O - 6 O - 6

Balance the Fe

4Fe + 3O₂ --> 2Fe₂O₃

Fe - 4 Fe - 4

O -6 O - 6

Tags

NGSS.HS-PS1-7

When the equation

_C₂H₄ + _O₂ --> _CO₂ + _H₂O

is balanced using the smallest whole numbers, what is the coefficient of the O₂?

• 1

• 2

• 3

• 4

Answer explanation

Balance the reaction:

C₂H₄ + O₂ --> CO₂ + H₂O

Left Side: Right Side:

C- 2 C- 1

H- 4 H- 2

O- 2 O - 2 + 1 = 3

This is a bit harder because the right side has O in 2 different places.

Tip: Start with the side that has the O in 2 places. Make it so that there is an even number of O by adding a 2 in front of H₂O

C₂H₄ + O₂ --> CO₂ + 2H₂O

Left Side: Right Side:

C- 2 C- 1

H-4 H-4

O-2 O- 2 + 2 = 4

Balance the C

C₂H₄ + O₂ --> 2CO₂ + 2H₂O

Left Side: Right Side:

C- 2 C- 2

H- 4 H- 4

O-2 O- 4 + 2 = 6

Balance the O

C₂H₄ + 3O₂ --> 2CO₂ + 2H₂O

Tags

NGSS.HS-PS1-7

What is the gram-formula mass of Al₂(SO₄)₃?

• 123 g/mol

• 150 g/mol

• 214 g/mol

• 342 g/mol

Answer explanation

Use the Periodic Table

Al₂(SO₄)₃

1) Find number of atoms

Al - 2

S- 3

O-12

2) Mutliply by the mass of each atom.

Al - 2 x 27 = 54

S - 3 x 32 = 96

O - 12 x 16 = 192

3) Add total mass of each atom.

54 + 96 + 192 = 342 g/mol

What is the percent composition by mass of nitrogen in (NH₄)₂CO₃ (gram-formula mass = 96 g/mol) ?

• 14.6%

• 29.2%

• 58.4%

• 87.5%

Answer explanation

Use the equation for % composition on the back of the reference table.

Mass of part = mass of element (Nitrogen)

Mass of whole = gram-formula mass

In the compound (NH₄)₂CO₃

Mass of N = 14 x 2 = 28 g

Gram-formula mass = 96 g

% N = (28g/96g) x 100 = 29.2%

What is the mass of 4.5 moles of KOH (gram-formula mass = 56 g/mol) ?

• 0.080 g
• 56 g
• 36 g
• 252 g

Answer explanation

Use the mole calculations equation on the back of the reference table.

4.5 mole = Mass / 56 g/mol

Mass = 4.5 mol x 56 g/mol

Mass = 252 g

Tags

NGSS.HS-PS1-7