Question 1

Which of the following best describes a 'formula unit' in the context of ionic compounds?

Accepted Answer

The simplest whole-number ratio of ions in an ionic solid that results in electrical neutrality.

Question 2

Given the ions $$\mathrm{Ba$$^{2+}$$}$$ and $$\mathrm{PO_4$^{3-}}$$, what is the correct formula unit for the resulting ionic compound?

Accepted Answer

$$\mathrm{Ba_3$(PO_4$)_2}$$

Question 3

Which of the following statements about crystal lattices in ionic compounds is correct?

Accepted Answer

A crystal lattice is a 3D arrangement of several cations and anions that forms a stable pattern.

Question 4

When writing the formula for an ionic compound formed from $$\mathrm{Al$$^{3+}$$}$$ and $$\mathrm{N$$^{3-}$$}$$, what is the correct formula and why?

Accepted Answer

$$\mathrm{AlN}$$, because the charges are equal and cancel each other out in a 1:1 ratio.

Question 5

Which of the following is the correct formula for the compound formed from $$\mathrm{Mg$$^{2+}$$}$$ and $$\mathrm{SO_4$^{2-}}$$?

Accepted Answer

$$\mathrm{$$MgSO_4$$}$$

Question 6

A student is asked to write the formula for sodium dichromate. If the ions are $$\mathrm{Na^+}$$ and $$\mathrm{Cr_2O_7$$^{2-}$$}$$, what is the correct formula?

Accepted Answer

$$\mathrm{Na_2Cr_2O_7}$$

Question 7

Why do ionic compounds form crystal lattices rather than existing as discrete molecules?

Accepted Answer

Because the electrostatic attraction between oppositely charged ions extends in all directions, creating a repeating 3D pattern.

Question 8

If a compound is formed from $$\mathrm{Ca$$^{2+}$$}$$ and $$\mathrm{Cl^-}$$, what is the correct formula unit and how many chloride ions are needed to balance the charge?

Accepted Answer

$$\mathrm{$$CaCl_2$$}$$; two chloride ions are needed to balance the $$+2$$ charge of calcium.

Question 9

Which of the following best explains why the formula for sodium chloride is $$\mathrm{NaCl}$$ and not $$\mathrm{Na_2Cl_2}$$?

Accepted Answer

Because the formula unit represents the lowest whole-number ratio of ions that results in a neutral compound.

Question 10

A patient is given a medication containing $$\mathrm{K_2SO_4}. $$What ions are present in this compound, and in what ratio?

Accepted Answer

Two $$\mathrm{K^+}$$ ions for every one $$\mathrm{SO_4$^{2-}}$$ ion.

Question 11

Which of the following is the correct process for writing the formula of an ionic compound when the charges of the ions are different?

Accepted Answer

Cross the charges to become the subscripts for the opposite ion, then reduce to the lowest whole-number ratio.

Question 12

Which of the following compounds is correctly matched with its formula unit?

Accepted Answer

Magnesium sulfate: $$\mathrm{$$MgSO_4$$}$$

Question 13

A chemist needs to prepare a sample of aluminum sulfide. If the ions are $$\mathrm{Al$$^{3+}$$}$$ and $$\mathrm{S$$^{2-}$$}$$, what is the correct formula unit?

Accepted Answer

$$\mathrm{Al_2S_3}$$

Question 14

Which of the following best describes why ionic compounds are electrically neutral?

Accepted Answer

Because the total positive charge from the cations equals the total negative charge from the anions.

Question 15

If a compound is formed from $$\mathrm{Fe$$^{3+}$$}$$ and $$\mathrm{O$$^{2-}$$}$$, what is the correct formula unit?

Accepted Answer

$$\mathrm{Fe_2O_3}$$

Question 16

A student writes the formula for calcium nitrate as $$\mathrm{$$CaNO_3$$}. $$What is the error in this formula?

Accepted Answer

The correct formula is $$\mathrm{Ca(NO_3$)_2}$$ because two nitrate ions are needed to balance the $$+2$$ charge of calcium.

Formula Units and Writing Ionic Compound Formulas

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