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Chemistry - Solutions

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    Study Guide Chemistry – Solutions 1. Concentration Units 1. 1 What Is a Solution? A solution is a mixture of two or more substances that has the same composition throughout . Every solution has two main parts: • Solvent – the host substance present in the larger amount • Solute – the substance that is dissolved in the solvent Most familiar solutions are liquids, such as salt dissolved in water. However, solutions are not limited to liquids . They can also be gases or solids. 1. 2 Variable Composition of Solutions In a solution, the ratio of solvent to solute is not fixed . This means the amount of solute can change without changing the identity of the solution. This behavior is different from chemical compounds , which always contain elements in fixed and definite ratios . 1. 3 Different Types of Solutions • Seawater o Solvent: water o Solutes: sodium chloride, calcium carbonate, magnesium bromide, and others o The concentration of each solute can vary • Carbonated soda water o Solvent: water o Solute: carbon dioxide gas

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    Study Guide • Air o Solvent: nitrogen (present in the largest amount) o Solute: oxygen (present in a smaller amount) 1. 4 Solid Solutions: Electrum A solid solution is formed when solids mix evenly. An example is electrum , an alloy made of gold and silver . • The ratio of gold to silver is not fixed • The metal present in the larger amount is the solvent • The metal present in the smaller amount is the solute 1. 5 Meaning of Concentration The concentration of a solution describes how much solute is present compared to the solvent. Chemists use several different units to express concentration. These units are important because accurate description of solutions is essential in chemistry and laboratory work. 1. 6 Mass Percent One way to express concentration is mass percent . Mass Percent Formula Mass percent = (mass of component ÷ total mass of solution) × 100% Example: Mass Percent in Electrum An electrum ingot is made by mixing: • 62 g of gold • 800 g of silver Total mass of the alloy: 62 g + 800 g = 862 g

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    Study Guide Mass Percent of Gold Mass percent of gold = (62 g ÷ 862 g) × 100% Mass percent of gold = 7.2% Mass Percent of Silver Mass percent of silver = (800 g ÷ 862 g) × 100% Mass percent of silver = 92.8% 1. 7 Importance of Atomic Mass Gold and silver have different atomic masses . Because of this, mass percent does not show the actual number of atoms present. To find how many atoms of each element are present, we must calculate the number of moles . Figure 1. Silver and gold. 1. 8 Calculating Number of Moles Moles of Silver Moles of Ag = mass of Ag ÷ molar mass of Ag Moles of Ag = 800 g ÷ 107.87 g/mol Moles of Ag = 7.416 moles

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    Study Guide 1 .9 Moles of Gold Moles of Au = mass of Au ÷ molar mass of Au Moles of Au = 62 g ÷ 196.97 g/mol Moles of Au = 0.315 moles Total moles in the alloy: 7.416 + 0.315 = 7.731 moles 1. 10 Mole Fraction The mole fraction shows the fraction of total particles that belong to each component. Mole Fraction Formula Mole fraction of a component (X) = moles of component ÷ total moles of all components Mole Fraction of Silver X ₍ Ag ₎ = 7.416 ÷ 7.731 X ₍ Ag ₎ = 0.959 This means 959 out of every 1,000 atoms are silver. Mole Fraction of Gold X ₍ Au ₎ = 0.315 ÷ 7.731 X ₍ Au ₎ = 0.041 This means 41 out of every 1,000 atoms are gold. The sum of mole fractions in a solution is always equal to 1.00 . 1. 1 1 Common Concentration Units for Liquid Solutions Mass percent and mole fraction can be used for all types of solutions. However, the most commonly used concentration units for liquid solutions are molarity and molality . If water is the solvent, the solution is called an aqueous solution .

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    Study Guide 1 .1 2 Molarity (M) Molarity is defined as the number of moles of solute present in 1 liter of solution . Molarity Formula Molarity (M) = moles of solute ÷ liters of solution Example: Calcium Chloride Solution A 0.5 M CaCl ₂ solution contains: • 0.5 moles of CaCl ₂ • Enough water to make the total volume 1 liter Molarity is the most commonly used unit in laboratory work. 1. 1 3 Molality (m) Molality is defined as the number of moles of solute present in 1 kilogram of solvent . Molality Formula Molality (m) = moles of solute ÷ kilograms of solvent Example: Hydrogen Fluoride Solution A 2 m HF solution contains: • 2 moles of HF • Dissolved in 1.0 kg (1,000 g) of water Molality is preferred for certain calculations because it does not depend on volume. 1. 1 4 Practice Problems 1. Molality Problem 2. 80 g of glucose (C ₆ H ₁₂ O ₆ ) is added to 750 g of water. 3. Calculate the molality of glucose in the solution. 4. Mole Fraction Problem 5. A solution is prepared by mixing 100 g of methyl alcohol (CH ₃ OH) and 100 g of water. 6. Calculate the mole fraction of methyl alcohol.

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    Study Guide 2. Quiz: Concentration Units 1. Question A homogeneous mixture of two or more substances is a(n) Answer Choices • Compound • Solution • Element Correct Answer Solution Why This is Corre ct A solution is a homogeneous mixture, which means it has the same composition throughout . In a solution, the substances are evenly mixed, so you cannot see the different components separately. For Example Salt dissolved in water forms a solution. The salt spreads evenly through the water, making the mixture uniform. 2. Question In a solution, the host substance is the Answer Choices • Solvent • Solute • Mixture Correct Answer Solvent

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    Study Guide Why This is Corre ct The solvent is the substance that is present in a larger amount and dissolves the other substance (solute) to form a solution. For Example In salt water, water is the solvent and salt is the solute. 3. Question The solute is the Answer Choices • host substance • solid phase only • dissolved substance Correct Answer dissolved substance Why This Is Corre ct A solute is the substance that gets dissolved in another substance to form a solution. The substance that does the dissolving is called the solvent , not the solute. For example: • In saltwater, salt is the solute because it dissolves. • Water is the solvent because it dissolves the salt. So, the correct definition of a solute is the dissolved substance . 4 . Question Measuring the relative mass of the constituents of a solution is known as

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    Study Guide Answer Choices • relative masses • mass percents • common mass Correct Answer mass percents Why This is Corre ct Mass percent is a concentration unit that expresses the relative mass of each component in a solution. It tells us how much of the solute is present compared to the total mass of the solution, multiplied by 100. For Example If 10 g of salt is dissolved in 90 g of water, the total mass of the solution is 100 g. Mass percent of salt = (10 ÷ 100) × 100 = 10% 5. Question A solution is made by mixing 250.0 g of hexane and 50.0 g of octanol. What is the mass percent of the octanol? Answer Choices • 83.3% • 16.7% • 20.0% Correct Answer 16.7%

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    Study Guide Why This is Corre ct Mass percent is calculated using the formula: Here, octanol is the solute. Total mass of solution = 250.0 g + 50.0 g = 300.0 g For Example If 10 g of salt is mixed with 90 g of water, the mass percent of salt is: (10/100) × 100 = 10% 6 . Question A solution of sodium chloride and water consists of 3.0 moles of sodium chloride and 7.0 moles of water . What is the mole fraction of sodium chloride ? Answer Choices • 0.43 • 0.70 • 0.30 Correct Answer 0.30 Why This is Corre ct The mole fraction of a substance is calculated by dividing the number of moles of that substance by the total number of moles in the solution .

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    Study Guide For Example If a class has 3 boys and 7 girls, the fraction of boys is 3/10 = 0.30 7. Question The number of moles of solute per kilogram of solvent is known as Answer Choices • molarity • molality • solubility Correct Answer molality Why This Is Corre ct Molality is defined as the number of moles of solute per kilogram of solvent . It is different from molarity, which depends on the volume of the solution. For example: If 1 mole of salt is dissolved in 1 kilogram of water, the solution has a molality of 1 m . Molality is especially useful because it does not change with temperature , making it reliable for scientific calculations. 8 . Question The term aqueous solution means the

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