Stoichiometry Test Review

B₂H₆ + 3O₂ -->2 HBO₂ + 2 H₂O
 What mass in grams of O₂ will be needed to burn 36.1 g of B₂H₆?

• 13.8 g O₂

• 3.86 mol of O₂

• 124 g O₂

Answer explanation

To find the mass of O2 needed, first calculate moles of B2H6: 36.1 g / 27.67 g/mol = 1.30 mol. The reaction shows 1 mol B2H6 requires 3 mol O2, so 1.30 mol B2H6 needs 3.90 mol O2. Converting to grams: 3.90 mol * 32 g/mol = 124 g O2.

Tags

NGSS.HS-PS1-7

Using the following equation:
Fe₂O₃(s) + 3H₂(g) → 2Fe(s) + 3H₂O(l)
How many moles of iron, Fe, can be made from  6 moles H₂? 

• 4 moles Fe

• 6 moles Fe

• 9 moles Fe

• 2 moles Fe

Answer explanation

From the equation, 3 moles of H2 produce 2 moles of Fe. With 6 moles of H2, we can calculate: (6 moles H2) x (2 moles Fe / 3 moles H2) = 4 moles Fe. Thus, the correct answer is 4 moles Fe.

Tags

NGSS.HS-PS1-7

9. Complete the equation for the percent yield of a chemical reaction:
Percent yield=(________)÷(________)×100%

• actual yield; theoretical yield

• theoretical yield; actual yield

Answer explanation

The percent yield of a chemical reaction is calculated by dividing the actual yield by the theoretical yield and then multiplying by 100%. Thus, the correct completion is: Percent yield=(actual yield)÷(theoretical yield)×100%.

Tags

NGSS.HS-PS1-7

What is an excess reactant?

• amount you end with

• what you run out of first

• what you have left over

• what you start with

Answer explanation

An excess reactant is the substance that remains after a chemical reaction has completed. It is what you have left over, as opposed to the limiting reactant, which is consumed first.

2 KClO₃ → 2 KCl + 3 O₂
How many moles of oxygen, O₂, are produced when 6.7 moles of KClO₃ decompose completely?

• 6.7 mol

• 1.0 mol

• 10.1 mol

• 4.5 mol

Answer explanation

From the balanced equation, 2 moles of KClO3 produce 3 moles of O2. Therefore, 6.7 moles of KClO3 will produce (6.7 moles KClO3) * (3 moles O2 / 2 moles KClO3) = 10.1 moles O2.

Tags

NGSS.HS-PS1-7

When does a chemical reaction stop?

• When the lab is finished

• When the excess reactant is used up

• When the limiting reactant is used up

• Chemical reactions never stop

Answer explanation

A chemical reaction stops when the limiting reactant is used up, as it determines the maximum amount of product that can be formed. Once it is consumed, the reaction cannot proceed further.

Tags

NGSS.HS-PS1-6

What is the mole ratio of H₂O to H₃PO₄ in the following chemical equation?   P₄O₁₀ + 6 H₂O --> 4 H₃PO₄ 

• 6:4

• 4:6

• 1:3

• 3:1

Answer explanation

In the equation P4O10 + 6 H2O -> 4 H3PO4, the coefficients indicate that 6 moles of H2O react with 4 moles of H3PO4. Thus, the mole ratio of H2O to H3PO4 is 6:4, which is the correct answer.

Tags

NGSS.HS-PS1-7

You need 2 pieces of bread, 1 tablespoon of peanut butter and 2 tablespoons of jelly to make a sandwich.  If you have 10 pieces of bread, 4 tablespoons of peanut butter and 20 tablespoons of jelly, what is the limiting reactant?

• bread

• jelly

• peanut butter

• sandwich

Answer explanation

To make one sandwich, you need 1 tablespoon of peanut butter. With 4 tablespoons available, you can make 4 sandwiches. However, you can only make 5 sandwiches with the bread and jelly. Thus, peanut butter is the limiting reactant.

The Formula to make S'mores is:
2C + 1M + G₂ → C₂MG₂. If you have 5 Marshmallows (M), 9 Chocolate Pieces (C), and 9 Graham Cracker Halves (G), how many S'mores can you make?

• 5

• 4

• 8

• 42

Answer explanation

To make 1 S'more, you need 2C, 1M, and 2G. With 9C, 5M, and 9G, you can make: 4 S'mores (limited by M: 5/1 = 5, C: 9/2 = 4, G: 9/2 = 4). Thus, the maximum is 4 S'mores.

2Al + 3H₂SO₄ -> Al₂(SO₄)₃ + 3H₂
How many grams of aluminum sulfate, Al₂(SO₄)₃, would be formed if 250g H₂SO₄ completely reacted with aluminum?

• 0.85 g

• 290 g

• 450 g

• 870 g

Answer explanation

To find the mass of Al2(SO4)3 formed, first calculate moles of H2SO4: 250g / 98.08 g/mol = 2.55 mol. The reaction ratio shows 3 moles of H2SO4 produce 1 mole of Al2(SO4)3. Thus, 2.55 mol H2SO4 yields 0.85 mol Al2(SO4)3, which is 290g.

Tags

NGSS.HS-PS1-7