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# Regents Chemistry Practice Multiple Choice Questions and Solutions: Stoichiometry

January 2023

32 The table below shows the atomic masses and natural abundances of the two naturally occurring isotopes of rhenium. Which numerical setup can be used to calculate the atomic mass of rhenium?

1) (184.95 u)(37.40) + (186.96 u)(62.60)

(2) (184.95 u)(0.3740) + (186.96 u)(0.6260)

(3) ((184.95 u)(37.40) + (186.96 u)(62.60))/ 2

(4) ((184.95 u)(0.3740) + (186.96 u)(0.6260))/ 2

Solution: To calculate the average atomic mass of an element, we take the atomic mass of each isotope and multiply by percent abundance in the decimal form ( divide by 100).

Average atomic mass of rhenium = (184.95 u)(0.3740) + (186.96 u)(0.6260)

35 Given the balanced equation representing a reaction: 2KClO3 + energy→2KCl + 3O2

What is the mass of KCl produced when 24.51 grams of KClO3 reacts completely to produce 9.60 grams of O2?

(1) 5.31 g

(2) 14.91 g

(3) 34.11 g

(4) 43.71 g

Solution: According to the conservation of mass, mass of the reactants must be equal to the mass of products. We have all the masses except for KCl. Let's plug and chug: 24.51g = 9.60g + x

x = 14.91 g

36 Which equation represents conservation of atoms?

(1) TiO2 + 2Al → 2Al2O3 + Ti

(2) TiO2 + 4Al → 2Al2O3 + Ti

(3) 3TiO2 + 2Al → 2Al2O3 + 3Ti

(4) 3TiO2 + 4Al → 2Al2O3 + 3Ti

Solution: We need to make sure atoms of each element are balanced on each side. In answer choice 4, there are 3 Ti on both sides, 4 Al on both sides and 6 O on both sides.

37 One mole of bromine gas, Br2, has a mass of

(1) 35.0 g (2) 70.0 g (3) 79.9 g (4) 159.8 g Solution: Gram formula mass (or molar mass) gives us the number of grams per 1 mol of compound. Looking at the Periodic Table, bromine has mass of 79.904g per mol of Br. Br2 mass would be: 2* 79.904g = 159.8 g which is the correct answer

August 2022

36. Given the equation representing a reaction:

2H2(g) + 2NO(g) → N2(g) + 2H2O(g)

What is the mass of N2(g) produced when 1.0 gram of H2(g) completely reacts with 15.0 grams of NO(g) to produce 9.0 grams of H2O(g)?

(1) 7.0 g

(2) 14.0 g

(3) 25.0 g

(4) 28.0 g

Explanation: The law of conservation of mass tells us that mass is neither created nor destroyed in a reaction. Therefore, the mass of products must equal to the mass of reactants. We have all the masses expect for nitrogen gas. Let's say x is the mass of nitrogen gas.

1g+ 15g = X + 9g

X=7g

January 2020

38 Which equation represents conservation of charge? Solution: Conservation of charge means that the total charge to the left of the arrow must be equal to the total charge to the right of the arrow. In equation 2, we have two -1 charges on the left (total charge is -2) and 2e- on the right (total charge is -2).

August 2019

35 Which equation represents a conservation of atoms? (1) 2Fe+2O2 →Fe2O3

(2) 2Fe + 3O2 → Fe2O3

(3) 4Fe + 2O2 → 2Fe2O3

(4) 4Fe + 3O2 → 2Fe2O3

Solution: Conservation of atoms means there is an equal number of atoms of the same element on both sides. Please note that a coefficient in front of the molecule gets distributed to all the atoms in that molecule. For example 2Fe2O3 means there are 4 Fe atoms (2*2) and 6 O atoms (2*3).

In equation 4 we have, 4 Fe on both sides, and 6 O on both sides.

June 2019

36 Given the balanced equation for the reaction of butane and oxygen:

2C4H10 + 13O2 → 8CO2 + 10H2O + energy

How many moles of carbon dioxide are produced when 5.0 moles of butane react completely?

(1) 5.0 mol

(2) 10. mol

(3) 20. mol

(4) 40. mol

Solution: The coefficients in the equation are 2 for butane and 8 for carbon dioxide. This means for every 2 moles of butane reacted, 8 moles of carbon dioxide will be produced. For 5 moles of butane reacted (5moles of butane*8 mol CO2/ 2 moles butane) moles of CO2 will be produced

37 What is the percent composition by mass of nitrogen in the compound N2H4 (gram-formula mass = 32 g/mol)?

(1) 13%

(2) 44%

(3) 88%

(4) 93%

Solution: The mass of nitrogen on the Periodic Table is 14 g/mol. There are 2 atoms of nitrogen in the formula. We do (2*14g/mol / 32g/mol)*100% Answer: Choice 3

January 2019

35 Given the reaction: 2KClO3(s) 2KCl(s) + 3O2(g) How many moles of KClO3 must completely react to produce 6 moles of O2? (1) 1 mole

(2) 2 moles

(3) 6 moles

(4) 4 moles

Solution:The coefficients in the equation are 3 for O2 and 2 for KClO3. This means 3 moles of O2 will be produced when 2 moles of KClO3 are reacted. Therefore, for 6 moles of O2 which is twice the amount, we would get 2*2= 4moles of KClO3.

36 What is the number of moles of CO2 in a 220.-gram sample of CO2 (gram-formula mass = 44 g/mol)?

(1) 0.20 mol

(2) 5.0 mol (3) 15 mol

(4) 44 mol

Solution: Table T on the Reference Table shows the formula for mole calculations. number of moles = given mass/ gram-formula mass.

Number of moles. = 220g/44g/mol = 5.0moles

August 2018

35 The empirical formula for butene is

(1) CH2

(2) C2H4

(3) C4H6

(4) C4H8

Solution: An empirical formula is a formula with the simplest ratio of atoms. This means the subscripts in the formula can not be divide any further. For C2H4, we can divide both subscripts by 2 to get CH2, eliminating choice 2. For C4H6 we can divide both subscripts by 2 to get C2H3, eliminating choice 3. For choice 4, C4H8, we can divide both numbers by 4, eliminating it as well. CH2 is the only molecule that cannot be divided any further.

36 Which equation represents a conservation of charge? Solution: Conservation of charge means that the total charge to the left of the arrow must be equal to the total charge to the right of the arrow. In equation 4, we see

3*(+3)=+9 charge on the left side and 3*(+2) +3 = +9 on the right side

January 2018

36 What is the number of moles of KF in a 29-gram sample of the compound? (1) 1.0 mol

(2) 2.0 mol

(3) 0.50 mol

(4) 5.0 mol

Solution: Table T on the Reference Table shows the formula for mole calculations.

Number of moles = given mass/ gram-formula mass.

Let's first find the gram formula mass of KF. We need to find the mass of K and F on the Periodic table.

K=39 g/mol

F= 19 g/mol

Gram formula mass of KF = 39+19 = 58g/mol

Number of moles = 29g/58g/mol= 0.50 mol

June 2018

37 Given the equation representing a reaction: 2C + 3H2 → C2H6 What is the number of moles of C that must completely react to produce 2.0 moles of C2H6? (1) 1.0 mol

(2) 2.0 mol

(3) 3.0 mol

(4) 4.0 mol

Solution: The coefficients in the equation are 2 for C and 1 for C2H6 (if the coefficient is not shown, we can assume that it is 1). This means when 2 moles of C react they produce 1 mole of C2H6. Therefore, to produce 2.0 moles of C2H6, we would need twice the amount of C, which would be 2*2 = 4moles

June 2017

35 Given the balanced equation representing a reaction: 2Na(s) + Cl2(g) → 2NaCl(s) + energy If 46 grams of Na and 71 grams of Cl2 react completely, what is the total mass of NaCl produced? (1) 58.5 g

(2) 117 g (3) 163 g

(4) 234 g

Solution: According to the conservation of mass, mass of the reactants must be equal to the mass of products. Therefore mass of Na + mass of Cl2 = mass of NaCl

46g+71g = mass of NaCl

117 g = mass of NaCl

36 Given the balanced equation representing a reaction: 2NO + O2 → 2NO2 + energy

The mole ratio of NO to NO2 is

(1) 1 to 1

(2) 2 to 1

(3) 3 to 2

(4) 5 to 2

Solution: The coefficients in the equation represent mole ratios. The coefficient for NO is 2 and for NO2 is 2. The mole ratio would be 2 to 2 or 1 to 1 when simplified.

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