Learn what a solution is and how it is formed. Learn how to express the concentration of a solution in terms of molarity, molality and mass percent. Discover the differences between an electrolyte and a nonelectrolyte.

Solutions

Most things in your daily life are mixtures. Milk is a mixture. Shampoo is a mixture. Air is a mixture. A mixture is two or more substances that are mixed but are not chemically combined.

There are two different kinds of mixtures: heterogeneous and homogenous. In a heterogeneous mixture like salsa or chicken noodle soup, the different substances are distinguishable from one another. In homogenous mixtures, the different substances inside are indistinguishable from one another. The physical properties of a homogenous mixture like air or Gatorade are uniform.

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Solutions are homogenous mixtures made of a solute and a solvent. Solutes are substances that get dissolved. Solvents are the substances that do the dissolving.

Solutions can be made of any two phases of matter. Solutions may be made up of gases dissolved in gases, or solids dissolved in liquids, or liquids dissolved in liquids. Most of the time, solutions are made up of a solid solute and liquid solvent. Water is often referred to as the universal solvent because it is capable of dissolving so many different substances. In this lesson, we will be looking at examples of solutions made of solid solutes and water as the solvent.

One of the most important qualities of a solution is its concentration. A solution isn’t chemically combined, so it can have different amounts of solute dissolved in the solvent. More solute per solvent means a more concentrated solution. Similarly, less solute per solvent means a less concentrated solution. The concentration of a solution is most frequently described in terms of molarity. Molarity is the number of moles of a solute per liter of solution. It has the units M or mol/L.

Concentration of a solution may also be described in terms of molality. Molality is the mass of the solute per kilogram of solvent.



The unit for molality is an italicized, lowercase m. Molality is useful when performing calculations involving the boiling or freezing points of solutions.

Lastly, solution concentration can also be described by mass percent, which is the percent by mass of the solute in the solution. Mass percent can be calculated by dividing the mass of the solute by mass of solution and multiplying the result by 100%. Mass percent is often used in analysis of solutions, like water quality reports.

Calculating Molarity, Molality and Mass Percent

Imagine you work for a chemical supply company, and for every solution you make, you must give a report on concentration in terms of molarity, molality and mass percent.

Today, you have to dissolve 85.58 g of sucrose (table sugar) in 1.5 L of water. The molar mass of sucrose is 342.3 g per mole.

The first task on our list is to calculate molarity for this solution using the molarity equation:


molarity


We have liters of solution given to us but must calculate number of moles given both the mass of sucrose and the molar mass of sucrose.

To find moles of sucrose, we divide the given mass of sucrose by the molar mass of sucrose:



Now, we divide our number of moles solute by our liters of solution to find molarity:



Our next task is to find molality using our molality equation:



We know that we have 0.25 moles of solute – we just figured out that in the previous problem – and 1.5 liters of solvent. We must convert liters solvent to kilogram solvent. The density of water is 1 kg/L, meaning that the mass of one liter of water is equal to one kilogram. We have 1.5 L of water, equal to 1.5 kg of water.

Now we divide our number of moles solute by kilograms of solvent to find molality:


molality1


To determine the mass percent of the same solution, we would use the following equation:



For this, it is critical to have the same units for mass for both the solute and the solution. I’m going to use grams.

We have 85.58 grams of sucrose and 1.5 kg of water. There are 1,000 g in 1 kg, so we have 1500 g of water. Notice that we add the mass of the solvent to the mass of the solute to find the total mass of the solution. Now we plug this information into our equation and solve.


masspercent1


We’ve finished our report and can move on to the next item.

Electrolytes and Nonelectrolytes

The similarity between the term ‘electrolyte’ and ‘electricity’ is no coincidence! In the 1880s, Swedish chemist Svante Arrhenius recognized that some solutions could conduct electricity. He noted that the number of dissolved ions in solution is directly related to the strength of the solution’s electrical potential. Compounds that dissolve by breaking into ions and conduct electricity in solution are known as electrolytes. Most soluble salts, acids and bases are electrolytes.

Compounds that dissolve in water but do not conduct electricity are known as nonelectrolytes. Nonelectrolytes are often covalently bonded compounds like sugars or alcohols.

Not all electrolytes have the same strength. Electrolytes that are strong conductors of electricity are known as strong electrolytes. Examples of strong electrolytes include sodium chloride, hydrochloric acid and sodium hydroxide.

Other electrolytes that conduct electricity poorly are known as weak electrolytes. Substances such as acetic acid, citric acid or ammonia are weak electrolytes.

Ionic and Covalent Substances

As mentioned in the electrolyte segment, compounds that produce ions in solution are the best electrolytes. Compounds that contain ions are ionic compounds. Ionic compounds are neutrally charged compounds made of a positive metal ion bonded by electrostatic attraction to a negative nonmetal ion. Many ionic compounds can be dissolved in water. These ionic compounds are strong electrolytes. Other ionic compounds dissolve poorly in water. These ionic compounds are weak electrolytes.

It’s hard to predict whether or not an ionic compound will be a strong or weak electrolyte without a little help. This solubility rules table is designed to help determine if an ionic compound will dissolve in water or not. The first column shows an ion or group of ions. The second column indicates the general solubility of those ions, and the third column lists any exceptions to the general solubility of the ions in the first column. For example, all group 1 elements are soluble, no exceptions. Chlorides are usually soluble except when in the form of silver chloride (AgCl), mercury (II) chloride (Hg2 Cl2), or lead (II) chloride (PbCl2 ).

Ion Solubility Exceptions
Group 1 elements (Li, Na, K etc.) soluble none
Ammonium NH4 + soluble none
Nitrates NO3 soluble none
Chlorides Cl soluble except AgCl, Hg2 Cl2 and PbCl2
Sulfates SO4 -2 soluble none
Carbonates CO3 -2 insoluble except NH4 + and those formed from Group 1 elements
Hydroxides OH insoluble except those formed from Group 1 elements
Sulfides S-2 insoluble except NH4 + and those formed from Groups 1 and 2 elements

Nonelectrolytes are usually covalent compounds, which are formed when two or more nonmetals bond together by the sharing of electrons. While some of these compounds, like sugar, may dissolve well in water, they usually don’t dissociate to produce ions. Some covalent compounds like the weak electrolyte ammonia may cause other compounds to form ions, but ammonia itself does not dissociate.

Lesson Summary

A mixture is when two or more substances occupy the same space but are not chemically combined. A solution is a homogenous mixture of two or more substances.

One of the most important properties of a solution is its concentration. Concentration may be expressed in terms of:


molarity




Electrolytes are substances that dissolve by breaking into ions in solution and conduct electricity. Electrolyte solutions can conduct electricity. Solutions that dissolve in water but don’t conduct electricity are nonelectrolytes.

Ionic compounds are generally electrolytes. Covalent compounds are generally nonelectrolytes.

Learning Outcomes

After you have finished with this lesson, you’ll be able to:

  • Define mixture, solution, solute and solvent
  • Differentiate between heterogenous and homogenous mixtures
  • Explain the three ways to express concentration
  • Describe what nonelectrolytes and strong and weak electrolytes are
  • Summarize the differences in ionic versus covalent compounds