Q: Which of the following is the correct formula for calculating the average atomic mass of an element with two isotopes?

$$\text{Average mass} = (\text{mass}_1 \times \text{fraction}_1) + (\text{mass}_2 \times \text{fraction}_2)$$

Question 1

Which law states that the total mass of substances before and after a chemical reaction remains unchanged?

Accepted Answer

Law of Conservation of Mass

Question 2

A sample of magnesium (0.455 g) reacts with 2.315 g of oxygen gas. After the reaction, 2.015 g of oxygen remains unreacted. What is the mass of magnesium oxide formed?

Accepted Answer

$$0.755 	ext{ g}$$

Question 3

According to Dalton's atomic theory, which statement is FALSE?

Accepted Answer

Atoms can be created or destroyed in chemical reactions.

Question 4

Which experiment led to the discovery of the electron?

Accepted Answer

Cathode ray tube experiment

Question 5

A neutral atom of chlorine has an atomic number of 17 and a mass number of 35. How many neutrons does it contain?

Accepted Answer

$$18$$

Question 6

If a copper ion has 29 protons, 34 neutrons, and 27 electrons, what is its symbol?

Accepted Answer

$$^{63}	ext{Cu}^{2+}$$

Question 7

Which of the following best describes the Law of Definite Proportions?

Accepted Answer

A compound always contains the same elements in the same proportion by mass.

Question 8

Given the natural abundances and masses of magnesium isotopes: $$^{24}	ext{Mg} (78.99$$%, 23.985 g/mol), $$^{25}	ext{Mg} (10.00$$%, 24.986 g/mol), $$^{26}	ext{Mg} (11.01$$%, 25.983 g/mol), calculate the average atomic mass of magnesium.

Accepted Answer

$$24.31 	ext{ g/mol}$$

Question 9

How many atoms are present in $$2.35$$ mol of iron ($$	ext{Fe}$$)? Use Avogadro's number $$N_A = 6.022 	imes 10$$^{23}$$.

Accepted Answer

$$1.42 	imes 10$$^{24}$$

Question 10

A sample contains $$1.00 	imes 10$$^{22}$$ magnesium atoms. How many moles of magnesium does this represent?

Accepted Answer

$$0.016 	ext{ mol}$$

Question 11

Which statement about isotopes is correct?

Accepted Answer

Isotopes have the same number of protons but different numbers of neutrons.

Question 12

A compound contains 12 g of carbon and 32 g of oxygen. What is the ratio of oxygen to carbon by mass?

Accepted Answer

$$2:1$$

Question 13

Which of the following is NOT a fundamental particle in the atom?

Accepted Answer

Photon

Question 14

A sample of $$0.144 	ext{ g}$$ magnesium reacts with oxygen. If $$0.100 	ext{ g}$$ magnesium produces $$0.166 	ext{ g}$$ magnesium oxide, how much magnesium oxide will be produced from $$0.144 	ext{ g}$$ magnesium?

Accepted Answer

$$0.239 	ext{ g}$$

Question 15

Which of the following best explains why negative and positive particles are deflected in opposite directions in a magnetic field?

Accepted Answer

They have opposite electric charges.

Question 16

What is the molar mass of $$	ext{Al}_2(	ext{SO}_4)_3$$?

Accepted Answer

$$342 	ext{ g/mol}$$

Question 17

Which law is demonstrated when manganese forms three different oxides with oxygen in mass ratios of $$2:3:4$$?

Accepted Answer

Law of Multiple Proportions

Question 18

How many $$	ext{K}$$ atoms are present in $$371 	ext{ mg} of $$potassium, given that the natural abundance of $$^{40}	ext{K} is$$ $$0.012\%$$ and the molar mass of potassium is $$39.109 	ext{ g/mol}$$?

Accepted Answer

$$6.9 	imes 10$$^{17}$$

Question 19

Which of the following is the correct formula for calculating the average atomic mass of an element with two isotopes?

Accepted Answer

$$	ext{Average mass} = (	ext{mass}_1 	imes 	ext{fraction}_1) + (	ext{mass}_2 	imes 	ext{fraction}_2)$$

Atom and Atomic Theory: Foundations of Chemistry

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