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 115.8 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: thermometer correction at this temperature is : 6.6°C

percent recrystallization of benzoic acid recovery 67.5 %

Explanation:

PART A: If the thermometer is giving us a lecture below the real melting point of the substance, we must sum the correction value always to the laboratory melting point measured with this specifically thermometer.  

How we can get this correction value?

In this case, we rest the theorical value with the laboratory measured value: 122.4 °C -  115.8°C = 6.6°C

Our thermometer is 6.6°C below to the real value.

PART B. When calculating a percent recovery, we always  do a "rule of three". If 4.00g represents my initial mass (100%), then how much percentage represents 2.7g of final mass (?)

This is easy, just multiplying 2.7 x 100 and then we divide the result with 4 = 67.5 % of final recovery.