Question

Identify the compounds in this MCQ.?

Three compounds have the physical properties shown in the table.

`ulbb("compound"color(white)(mmmmmml)"P"color(white)(mmmm)"Q"color(white)(mml)"R"color(white)(mm))`
`"melting point/°C"color(white)(mmmll)2852color(white)(mmml)993color(white)(ml)"-119"`
`"boiling point/°C"color(white)(mmmm)3600color(white)(mmml)695color(white)(mml)39`
`"conductivity (solid)"color(white)(mmll)"poor"color(white)(mmm)"poor"color(white)(ml)"poor"`
`"conductivity (liquid)"color(white)(mm)"good"color(white)(mmm)"good"color(white)(ml)"poor"`
`"conductivity (aqueous)"color(white)(m)"insoluble"color(white)(m)"good"color(white)(m)"insoluble"`

What might be the identities of `"P, Q"` and `"R"`?

`ulbb(color(white)(mml)"P"color(white)(mm)"Q"color(white)(mm)"R"color(white)(mm))`
`"A"color(white)(m)"MgO"color(white)(m)"KCl"color(white)(m)"NH"_3`
`"B"color(white)(m)"MgO"color(white)(m)"NaF"color(white)(m)"C"_2"H"_5"Br"`
`"C"color(white)(m)"SiO"_2color(white)(m)"KCl"color(white)(ml)"C"_2"H"_5"Br"`
`"D"color(white)(m)"SiO"_2color(white)(m)"NaF"color(white)(m)"HCl"`

Answer 1

The answer is B.

Explanation:

Just by researching the melting point and boiling point of each compound, the identities of the three compounds could be determined. I looked these values up on Wikipedia.

`"MgO"`

m.p.: `2852^@"C"`

b.p.: `3600^@"C"`

`"NaF"`

m.p.: `993^@"C"`

b.p.: `1704^@"C"` `larr` close enough to `1695^@"C"`

`"C"_2"H"_5"Br"`

m.p.: `-120` to `-116^@"C"`

b.p.: `38.0` to `38.8^@"C"`

Answer 2

The correct answer is Option B.

Explanation:

Covalent compounds have low melting and boiling points, are often insoluble in water, and are poor conductors in aqueous solution.

However, `"HCl"` is a strong electrolyte. Its aqueous solution is a good conductor of electricity.

This observation eliminates Option D.

Both network solids and ionic compounds have high melting and boiling points. However, ionic compounds are good conductors in aqueous solution.

This observation eliminates Option C because `"SiO"_2"` is a network solid.

The correct option must be A or B. It cannot be A because ammonia is a gas at room temperature. Its boiling point must be less than 39 °C.

The correct answer is Option B.