Why are fats soluble in petroleum ether

Matthew Hessler

Natalie Price

Emily Morley

Nouf Alhodaibi

TEACHING NOTES for Determining Fat Content in Potato Chips

Brief Overview and Purpose

The primary focus of this lab is to find the fat content in potato chips. Petroleum ether will be used to extract the fat content from the potato chips. Students will determine the accuracy (percent error) and precision (average deviation) of their results by comparing to the known values on the nutrition labels.

Chemical Concepts and Calculations

Fat is not soluble in water however, it is soluble in organic solvents. Adding the organic solvent (petroleum ether) separates the other ingredients from the fat. This allows the fat to be filtered away from the other ingredients using a gravity filter. The solvent being used has a low boiling point which will allow the solvent to be boiled off while leaving the fat behind.

· Percentage of fat in sample (experimental value)

· Determining percentage of fat per serving (theoretical value)

· Conversion from ounce to grams

Students will need to measure one gram of potato chips. After experimental value is determined multiply by grams per respective serving size.

· Percent error

Procedures

· CAUTION: Petroleum ether is flammable. Only use inside fume hood.

1. Obtain 1 gram of specified potato chip and record the true value for mass.

2. Using a mortar and pestle crush the chips into a fine powder. (It can get sticky)

3. Add 10 mL of solvent (petroleum ether) to the ground up chips. Continue to grind up chips. The petroleum ether will separate fat from other ingredients.

4. Add filter paper to funnel. Pour mixture through funnel and into a 50 mL beaker. Filter paper should catch most other ingredients. Rinse pestle with ether as needed until all of mixture is out.

5. After the mixture is completely filtered, transfer the contents of beaker into separatory funnel and rinse out beaker with petroleum ether until all mixture is out.

6. Add water to separatory funnel. Invert slowly until mixture and water is homogeneous throughout. Let mixture settle into two layers.

7. Weigh 50 mL empty beaker and record mass. Drain bottom layer of petroleum ether and fat content mixture into beaker. Weigh beaker and record weight again.

8. Put beaker onto warm plate (not too hot) to evaporate petroleum ether until there is only a thick liquid left. Turn off hot plate, remove beaker and let cool. Weigh beaker and record.

Results

Students should expect fat content to be similar to theoretical value. Class can compare results to each other to determine precision calculations and record results on board.

Materials

Potato Chips

Mortar and pestle

Petroleum ether

Beakers

Separatory funnel

Water

Hot Plate

Funnel and Filter Paper

Safety

Petroleum ether is very flammable. Keep petroleum ether away from flames and conduct experiment under fume hoods. Do not eat during lab. Do not inhale petroleum ether. If you start to feel light headed step away from the hood or out in the hall. Wear gloves and goggles at all time.

Potential Errors

Sizes of potato chips vary and will cause variation of fat content throughout bag of chips. Manufacturer may have listed fat content higher than actual amount. Could be slight variation in calculations due to fat content given in ounces and measuring in grams. Petroleum ether might not be fully boiled off. Water and other ingredients could be in mixture after separating with separatory funnel. Some fat content could be lost from transferring to different containers.

Quiz questions

1) Why was petroleum ether used as the solvent instead of water?

A) Fat is not soluble in water.

B) Fat chemically reacts with water.

C) Petroleum ether is easier to evaporate.

2) What is the conversion ratio between ounces and grams?

A) 1:1

B) 1:89.4

C) 1:28.3495

3) Determine the % fat for the given values: mass of fat 2g total mass of food 5.3g?

A) 82%

B) 37.7%

C) 57.6%

4) What type of funnel is used to separate the petroleum ether mixture from the water mixture?

A) Buchner funnel

B) Hirsch funnel

C) Separatory funnel

5) If there are three ounces in a serving size how many grams are there in it?

A) 85.05g

B) .2552g

C) 0.1058g