U4-4 - Purity Assessment

Question 1

What type of melting point range will be seen for a pure substance?

Answer 1

Sharp melting point (melts over a 2 °C range or less)

Question 2

What type of melting point range will be seen for an impure substance?

(2 things)

Answer 2

Broad melting point, and lower than literature value.

Question 3

Why do impure substances have lower melting points?

Answer 3

Weaker intermolecular forces (due to presence of impurities)

Question 4

Mixed melting point analysis of sample X:

1. Mix sample X with a ________________.
2. If both are the same, melting point is the same as that of the pure substance.
3. If both are different, the melting point range will be broader.

Answer 4

Mixed melting point analysis of sample X:

1. Mix sample X with a pure sample of the same substance.
2. If both are the same, melting point is the same as that of the pure substance.
3. If both are different, the melting point range will be broader.

Question 5

Mixed melting point analysis of sample X:

1. Mix sample X with a pure sample of the same substance.
2. If both are the same, melting point is ___________.
3. If both are different, the melting point range will be broader.

Answer 5

Mixed melting point analysis of sample X:

1. Mix sample X with a pure sample of the same substance.
2. If both are the same, melting point is the same as that of the pure substance.
3. If both are different, the melting point range will be broader.

Question 6

Mixed melting point analysis of sample X:

1. Mix sample X with a pure sample of the same substance.
2. If both are the same, melting point is the same as that of the pure substance.
3. If both are different, _________.

Answer 6

Mixed melting point analysis of sample X:

1. Mix sample X with a pure sample of the same substance.
2. If both are the same, melting point is the same as that of the pure substance.
3. If both are different, the melting point range will be broader.

Question 7

How can boiling point be measured?

Answer 7

Using a distillation apparatus

Question 8

How can spots be visualised on a TLC plate?

Answer 8

Using a developing agent (e.g. ninhydrin) or UV light

Question 9

A TLC experiment was carried out, but the distances travelled by different spots were all similar.

What adjustment can be made to change this?

Answer 9

Use different solvent (or solvent mixture)

Question 10

How many spots will be produced by a pure substance in TLC?

Answer 10

One

Question 11

A compound is made and purified. In TLC, it produced two spots.

What does that say about the purity of the compound?

Answer 11

Impurities present

Question 12

How can the R_f value be calculated?

Answer 12