Question

You are asked to determine the thermometer correction for your assigned melting point apparatus. You obtain a pure sample of benzoic acid and determine its melting point range carefully. After several trials, you obtain a very sharp melting point of 115.1 degrees celsius. What is the thermometer correction at this temperature? For purposes of these questions, the melting point of very pure benzoic acid is 122.4 degrees Celsius. (The thermometer correction is the value that must be added to the observed thermometer reading. You have to type in your answer with a + or - sign. )In a recrystallization of benzoic acid, you begin with a sample of 4.00 grams. After several recrystallizations, a dry sample of 2.7 grams was obtained experimentally. What was your percent recovery? (The format of your answer is important, e.g. 33.2 percent would be entered as 33.2 without any percent symbol or the word percent.)

Answer 1

Answer:

Question 1: -7.3ºC

Question 2: 67.50%

Explanation:

Question 1:

For different reasons a thermometer can not give the exact temperature that it is measuring, such as incorrect calibration. So the thermometer will have an error, and for this, we can have a thermometer correction, doing experiments with a substance that we know the temperature. In this question that substance was benzoic acid.

The thermometer correction (Tc) is the temperature that was measured less the real temperature, so:

Tc = 115.1 - 122.4 = -7.3ºC

It means that the temperature given by the thermometer is 7.3ºC less than the real.

Question 2:

Recrystallization is the process of transforming the liquid in its solid form (crystals). So, the sample of 4.00 grams of benzoic acid, there'll be water or other impurities. The total benzoic acid given was 2.70 grams, so its percentage will be:

(mass of acid/ total mass)x100%

(2.70/4.00)x100% = 67.50%