What is the composition of a solution

Percent composition by mass is a statement of the percent mass of each element in a chemical compound or the percent mass of components of a solution or alloy. This worked example chemistry problem works through the steps to calculate percent composition by mass. The example is for a sugar cube dissolved in a cup of water.

A 4 g sugar cube (Sucrose: C12H22O11) is dissolved in a 350 ml teacup of 80 °C water. What is the percent composition by mass of the sugar solution?

Given: Density of water at 80 °C = 0.975 g/ml

Percent Composition by Mass is the mass of the solute divided by the mass of the solution (mass of the solute plus mass of the solvent), multiplied by 100.

Step 1 - Determine mass of solute

We were given the mass of the solute in the problem. The solute is the sugar cube.

masssolute = 4 g of C12H22O11

Step 2 - Determine mass of solvent

The solvent is the 80 °C water. Use the density of the water to find the mass.

density = mass/volume

mass = density x volume

mass = 0.975 g/ml x 350 ml

masssolvent = 341.25 g

Step 3 - Determine the total mass of the solution

msolution = msolute + msolvent

msolution = 4 g + 341.25 g

msolution = 345.25 g

Step 4 - Determine percent composition by mass of the sugar solution.

percent composition = (msolute / msolution) x 100

percent composition = ( 4 g / 345.25 g) x 100

percent composition = ( 0.0116) x 100

percent composition = 1.16%

Answer:

The percent composition by mass of the sugar solution is 1.16%

• It's important to remember you use the total mass of the solution and not just the mass of the solvent. For dilute solutions, this doesn't make a huge difference, but for concentrated solutions, you'll get a wrong answer.
• If you're given the mass of solute and mass of solvent, life is easy, but if you're working with volumes, you'll need to use density to find the mass. Remember density varies according to temperature. It's unlikely you'll find a density value corresponding to your exact temperature, so expect this calculation to introduce a small amount of error into your calculation.

A solution is a homogeneous mixture. That means the components of a solution are so evenly spread throughout the mixture that there are no perceivable differences in composition. Solutions can be formed by mixing two substances together such as sugar and water. If you pour a packet of sugar into a glass of water, initially you have a suspension as the sugar crystals float about in the glass. When you have stirred the sugar and water for long enough, you will eventually get a clear, colorless mixture. Some people, especially young children, can be fooled by such a demonstration into thinking that the sugar has "disappeared". However, as chemists, we know better. The law of conservation of matter states that the sugar can not just disappear, it must have gone somewhere else. That somewhere else is into solution. The sugar has become evenly dispersed. In fact the sugar molecules are so well spread out that we can no longer see a single sugar crystals. However, if you taste the water, you will find it to be sugary--confirming the presence of sugar in the water. The minor component of the solution is called the solute. In the present example, sugar is the solute. The major component of the solution is called the solvent. In this case water is the solvent.

Solutions can also be formed by mixing together many different phases of matter. For instance, air is a solution. The solute gasses oxygen, carbon dioxide, argon, ozone, and others are dissolved in the solvent nitrogen gas. Another example is found in "gold" jewelry. Most of the golden jewelry sold in the world is not 24 karat (i.e. 100% pure gold) but rather it is a solution of other metals, commonly silver and copper, in a gold solvent. Such a solution of metal(s) in another metal is called an amalgam.

The Composition of Solutions

Perhaps the most important property of a solution is its concentration. A dilute acetic acid solution, also called vinegar, is used in cooking while a concentrated solution of acetic acid would kill you if ingested. The only difference between such solutions is the concentration of the solute. In order to quantify the concentrations of solutions, chemists have devised many different units of concentration each of which is useful for different purposes.

Molarity, the number of moles of solute per liter of solution, has the units moles / L which are abbreviated M. This unit is the most commonly used measure of concentration. It is useful when you would like to know the number of moles of solute when you know both the molarity and the volume of a solution. For example, it is easy to calculate the volume of a 1.5 M solution of HCl necessary to completely react with 0.32 moles of NaOH:

Normality, the number of molar equivalents of solute per liter of solution, has the units equivalents / L which are abbreviated N. To illustrate the difference between molarity and normality let's assume that we had used a 1.5 M solution of sulfuric acid, H2SO4, instead of a 1.5 M solution of hydrochloric acid, HCl in the above example. Because sulfuric acid can donate two protons to the NaOH, as noted in the , it will only take half as much sulfuric acid as hydrochloric acid to neutralize the sodium hydroxide.

In the present example, the 1.5 M solution of sulfuric acid reacts like a 3.0 M solution of hydrochloric acid because there are two equivalents of H+ per mole of sulfuric acid. Therefore, that solution of sulfuric acid is 3.0 N.

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To be a solution, a mixture must be homogeneous--its components must be uniformly dispersed and separable only by chemical means. There are two parts of a solution, the solvent and the solutes. The solvent is what we call the major component and a solvent is one of the minor components. Solutions can be made between many phases of matter. For example, salt water is a solution of a solid solute (NaCl) in a liquid solvent (water) and air is a solution of a variety of gaseous solutes, including oxygen, in the gaseous solvent nitrogen.

To understand solutions we must first understand their compositions. To do so, we need to develop a quantitative way to talk about the concentration of a solution. Many different units of concentration have been developed. Mass percent is the mass of solute divided by the mass of the solution. Mole fraction is the number of moles of solute divided by the total number of moles of solvent. Molarity, perhaps the most common concentration unit, is the number of moles of solute divided by the number of liters of solution. Molality is the number of moles of solute divided by the number of kilograms of solvent. Normality is the number of molar equivalents of solute per liter of solution.

In addition to a discussion of the concentrations of solutions, we will briefly touch upon the ways in which solutions are made, including dilution.

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