Percent Water in Foods

Our bodies need water and our primary source of water is obtained by drinking fluids and eating food that are high in water content.  The amount of water in different foods can be determined by dehydrating these foods and measuring the amount of water lost as shown in the equation below:

% water = (mass of water lost/mass of food slices) x 100%

A solution can be described as concentrated or dilute. These terms are relative to one another and only tell you if a solution contains many particles of solute per particles of solvent (concentrated) or few particles of solute per particles of solvent (dilute). The percent concentration is a measurement used to tell us exactly how many particles of solute per solvent particles are present in a given solution. The following are expressions for percent concentration: mass/mass percent (m/m) and  mass/volume percent (m/v). Pay close attention to the units in each expression.
 

Mass/mass Percent	=	grams of solute	x	100
		grams of solution

 

Mass/volume Percent	=	grams of solute	x	100
		mL of solution

Molarity is another common way to express the concentration of a solution. The symbol for molarity is M. Molarity is equal to the number of moles of solute in one liter of solution.
 

M  =	moles of solute
	L of solution

Laboratory Activities

A. Percent Water in Various Foods

1. Preweigh a clean dry watch glass and record its mass to the nearest 0.01 g.

2. Cut several very thin pieces of one type of fruit or vegetable.  Place the slices of food on the preweighed watch glass and spread them out so that they are not overlapping.  Weigh the watch glass with the food slices on it and record the mass to the nearest 0.01 g.

3. Place the watch glass in the metal pan provided by your instructor.  This pan will be placed in a drying oven at 100-110 C for approximately one hour.  After the food sample is dehydrated, let the watch glass cool to room temperature.  Weigh and record the total mass of the watch glass and dried food sample to the nearest 0.01 g.

4.  Calculate the amount of water lost  when the food was dehydrated.

5.  Calculate the percent of water contained in your food sample by using the equation provided in the introduction.

6.  Share your data with at least two other students in the lab who used different foods and construct a table that clearly presents this data.  Compare these values and list the foods in order of increasing water content.
 
 
 

B. Determining the Concentration of a Sodium Chloride Solution

1.  Weigh a clean dry evaporating dish.  Record its mass to the nearest 0.01 g.

2.  Using a pipet pump and a 10-mL calibrated pipet, measure out 10.0 mL of a sodium chloride solution and place it in your preweighed evaporating dish. Record the volume of solution that you obtained and be sure to record which solution you are using (A, B, C or D).  Your instructor will show you the proper technique for using the pipet pump.

3.  Weigh the evaporating dish and the NaCl solution together.  Record the mass to the nearest 0.01 g.

4.  Place about 200 mL of water into a 250-mL beaker.  Place the evaporating dish on top of the beaker.  Heat the water in the beaker to boiling using a Bunsen burner.  As the water in the beaker evaporates, you may need to add more water.  Do not let all of the water boil out of the beaker.

5.  When the all of the water from the NaCl solution has evaporated from the evaporating dish, turn off the burner.  After the evaporating dish has cooled, dry the bottom and place the dish on an iron ring using a wire gauze.  Heat the dish gently with a low flame to dry the salt completely.  Allow the dish to cool.  Weigh the evaporating dish containing the dry NaCl and record its mass to the nearest 0.01 g.
 

Calculations

A.  Calculate the mass of the solution.

B.  Calculate the mass of the NaCl after drying.

C.  Calculate the mass/mass percent concentration using equation given below.
 

Mass/mass Percent	=	grams of dry NaCl	x	100
		grams of solution

D.  Calculate the mass/volume percent concentration using the following equation:
 

Mass/volume Percent	=	grams of dry NaCl	x	100
		mL of solution

E.  Using the mass of dried NaCl that you calculated in Calculation B, calculate the moles of NaCl.

F.  Convert the volume in mL of the solution to liters.

G.  Calculate the molarity of the NaCl solution using the following equation:
 

M  =	moles of NaCl
	L of solution

Issues to be addressed in your conclusion...

Briefly summarize your findings concerning the percent water in foods.  Did any of the results surprise you?

Where your results for the % (m/m) concentration and %(m/v) concentration for the salt solution similar?  Did you expect them to be?  Why or why not?

What did you determine the molarity of your NaCl solution to be?

How are molarity and percent concentration similar?  How are they different?