Solution composition molarity

Example 14.2 Solution Composition Determining Mass of Solute  

Although milk is not a true solution (it is really a suspension of tiny globules of fat, protein, and other substrates in water), it does contain a dissolved sugar called lactose. Cow s milk typically contains 4.5% by mass of lactose, C12H22O11. Calculate the mass of lactose present in 175 g of milk. 

Mass of solution (milk) = 175 g Mass percent of solute (lactose) = 4.5% 

We need to calculate the mass of solute (lactose) present in 175 g of milk. Using the definition of mass percent, we have 

We now solve for grams of solute by multiplying both sides by 175 g. 

EXERCISE 15.2 What mass of water must be added to 425 g of formaldehyde to prepare a 40.0% (by mass) solution of formaldehyde This solution, called formalin, is used to preserve biological specimens. 

HINT Substitute the known quantities into the definition for mass percent, and then solve for the unknown quantity (mass of solvent). 

To understand molarity. To learn to use molarity to calculate the number of moles of solute present. 

When a solution is desaibed in terms of mass percent, the amount of solution is given in terms of its mass. However, it is often more convenient to measure the volume of a solution than to measure its mass. Because of this, chemists often describe a solution in terms of concentration. We define the concentration of a solution as the amount of solute in a given volume of solution. The most commonly used expression of concentration is molarity (Af). Molarity describes the amount of solute in moles and the volume of the solution in liters. Molarity is the number of moles of solute per volume of solution in liters. That is 

A solution that is 1.0 molar (written as 1.0 AT) contains 1.0 mole of solute per liter of solution. 

Adding 1.0 mole of solute to 1.0 L of water does not make a 1.0 M solution. Why not  

We need to consider both the amount of solute and the volume when determining the concentration of a solution. For example, consider the following two solutions where each dot represents 1.0 mol of solute. 

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CHEMISTRY IN FOCUS Green Chemistry 480 Solution Composition An Introduction 480 Solution Composition Mass Percent 481 Solution Composition Molarity 483... 

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