Q: How many atoms are present in 2.50 mol of hydrogen atoms? (Avogadro’s number $$= 6.022 \times 10$$^{23}$$ mol$$^{-1}$$)

$$1.51 \times 10$$^{24}$$ atoms

Question 1

The element boron (B) has an atomic mass of 10.81 amu according to the periodic table. However, no single atom of boron has a mass of exactly 10.81 amu. What is the best explanation for this observation?

Accepted Answer

The atomic mass is a weighted average of the masses of all naturally occurring isotopes of boron.

Question 2

Which experimental result from Rutherford’s gold-foil experiment provided evidence for the existence of a small, dense nucleus in the atom?

Accepted Answer

A few alpha particles were deflected at large angles, indicating a dense, positively charged center.

Question 3

Ibuprofen, C$$_{13}H$_{18}O$_2$$, manufactured in Michigan contains 75.69% carbon, 8.80% hydrogen, and 15.51% oxygen by mass. If you buy ibuprofen in Germany, how do you know it has the same percentage composition?

Accepted Answer

The law of definite proportions states that a chemical compound always contains the same elements in the same proportions by mass, regardless of the source.

Question 4

Given the atomic masses Fe: 55.85, O: 16.00, what is the mass ratio of O to Fe in Fe$$_2O$_3$$?

Accepted Answer

$$\frac{48.00}{111.70}$$

Question 5

How many atoms are present in 2.50 mol of hydrogen atoms? (Avogadro’s number $$= 6.022 	imes 10$$^{23}$$ mol$$^{-1}$$)

Accepted Answer

$$1.51 	imes 10$$^{24}$$ atoms

Question 6

A sample contains 107 g of sodium (Na). How many moles of sodium are present? (Atomic mass of Na $$= 22.99 g/$$mol)

Accepted Answer

$$4.65$$ mol

Question 7

A certain element exists as three natural isotopes: 19.99244 amu (abundance 0.9051), 20.99395 amu (abundance 0.0027), and 21.99138 amu (abundance 0.0922). What is the average atomic mass of this element?

Accepted Answer

$$20.18$$ amu

Question 8

Which of the following best describes the law of multiple proportions?

Accepted Answer

When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in ratios of small whole numbers.

Question 9

Which statement about isotopes is correct?

Accepted Answer

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

Question 10

Which of the following is the correct electron configuration for a neutral atom of sodium (Na, atomic number 11)?

Accepted Answer

$$1s^2$$\ $$2s^2$$\ $$2p^6$$\ $$3s^1$$

Question 11

Which of the following statements best explains why atoms are electrically neutral?

Accepted Answer

The number of protons equals the number of electrons in a neutral atom.

Question 12

Which of the following best describes the difference between a compound and a mixture?

Accepted Answer

A compound has a fixed composition and properties, while a mixture can have variable composition and properties.

Question 13

Which of the following is the correct formula for calculating the number of moles ($$n) $$from a given mass ($$m) $$and molar mass ($$M$$)?

Accepted Answer

$$n = \frac{m}{M}$$

Question 14

A sample of Fe$$_2O$_3$$ contains 167.55 g of Fe and 48.00 g of O. What is the mass ratio of O to Fe in this sample?

Accepted Answer

$$\frac{48.00}{167.55}$$

Question 15

Which of the following best explains why the atomic mass of chlorine is not a whole number?

Accepted Answer

It is a weighted average of the masses of all naturally occurring isotopes of chlorine.

Question 16

Which of the following statements is consistent with Dalton’s atomic theory?

Accepted Answer

Atoms of the same element are identical in mass and properties.

Question 17

Which of the following best describes the result if a sample of a compound is broken down into its elements?

Accepted Answer

The elements are always found in the same proportion by mass as in the original compound.

Question 18

Which of the following is the correct number of protons, neutrons, and electrons in a neutral atom of $$^{23}_{11}Na$$?

Accepted Answer

11 protons, 12 neutrons, 11 electrons

Question 19

A sample of uranium contains 2.50 mol. How many atoms of uranium are present? (Avogadro’s number $$= 6.022 	imes 10$$^{23}$$ mol$$^{-1}$$)

Accepted Answer

$$1.51 	imes 10$$^{24}$$ atoms

Atoms: The Building Blocks of Matter – Study Notes

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