Chemistry - Solutions - Quick Guides

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Study GuideChemistry–Solutions1.Concentration Units1.1What Is a Solution?Asolutionis a mixture of two or more substances that has thesame 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 solventMost familiar solutions are liquids, such as salt dissolved in water. However, solutions arenot limitedto liquids. They can also be gases or solids.1.2Variable Composition of SolutionsIn a solution, the ratio of solvent to solute isnot fixed.This means the amount of solute can change without changing the identity of the solution.This behavior is different fromchemical compounds, which always contain elements infixed anddefinite ratios.1.3Different Types of Solutions•SeawateroSolvent: wateroSolutes: sodium chloride, calcium carbonate, magnesium bromide, and othersoThe concentration of each solute can vary•Carbonated soda wateroSolvent: wateroSolute: carbon dioxide gas

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Study Guide•AiroSolvent: nitrogen (present in the largest amount)oSolute: oxygen (present in a smaller amount)1.4Solid Solutions: ElectrumAsolid solutionis formed when solids mix evenly.An example iselectrum, an alloy made ofgold and silver.•The ratio of gold to silver isnot fixed•The metal present in the larger amount is thesolvent•The metal present in the smaller amount is thesolute1.5Meaning of ConcentrationTheconcentrationof a solution describeshow much soluteis present compared to the solvent.Chemists use several different units to express concentration. These units are important becauseaccurate description of solutions is essential in chemistry and laboratory work.1.6Mass PercentOne way to express concentration ismass percent.Mass Percent FormulaMass percent = (mass of component ÷ total mass of solution) × 100%Example: Mass Percent in ElectrumAn electrum ingot is made by mixing:•62 g of gold•800 g of silverTotal mass of the alloy:62 g + 800 g = 862 g

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Study GuideMass Percent of GoldMass percent of gold = (62 g ÷ 862 g) × 100%Mass percent of gold = 7.2%Mass Percent of SilverMass percent of silver = (800 g ÷ 862 g) × 100%Mass percent of silver = 92.8%1.7Importance of Atomic MassGold and silver havedifferent atomic masses.Because of this, mass percent doesnotshow the actual number of atoms present.To find how many atoms of each element are present, we must calculate thenumber of moles.Figure1.Silver and gold.1.8Calculating Number of MolesMoles of SilverMoles of Ag = mass of Ag ÷ molar mass of AgMoles of Ag = 800 g ÷ 107.87 g/molMoles of Ag = 7.416 moles

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Study Guide1.9Moles of GoldMoles of Au = mass of Au ÷ molar mass of AuMoles of Au = 62 g ÷ 196.97 g/molMoles of Au = 0.315 molesTotal moles in the alloy:7.416 + 0.315 = 7.731 moles1.10Mole FractionThemole fractionshows the fraction of total particles that belong to each component.Mole Fraction FormulaMole fraction of a component (X) = moles of component ÷ total moles of all componentsMole Fraction of SilverX₍Ag₎= 7.416 ÷ 7.731X₍Ag₎= 0.959This means959 out of every 1,000 atomsare silver.Mole Fraction of GoldX₍Au₎= 0.315 ÷ 7.731X₍Au₎= 0.041This means41 out of every 1,000 atomsare gold.The sum of mole fractions in a solution is always equal to1.00.1.11Common Concentration Units for Liquid SolutionsMass percent and mole fraction can be used for all types of solutions.However, the most commonly used concentration units for liquid solutions aremolarityandmolality.If water is the solvent, the solution is called anaqueous solution.

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Study Guide1.12Molarity (M)Molarityis defined as the number of moles of solute present in1 liter of solution.Molarity FormulaMolarity (M) = moles of solute ÷ liters of solutionExample: Calcium Chloride SolutionA 0.5 M CaCl₂solution contains:•0.5 moles of CaCl₂•Enough water to make the total volume 1 literMolarity is the most commonly used unit in laboratory work.1.13Molality (m)Molalityis defined as the number of moles of solute present in1 kilogram of solvent.Molality FormulaMolality (m) = moles of solute ÷ kilograms of solventExample: Hydrogen Fluoride SolutionA 2 m HF solution contains:•2 moles of HF•Dissolved in 1.0 kg (1,000 g) of waterMolality is preferred for certain calculations because it does not depend on volume.1.14Practice Problems1.Molality Problem2.80 g of glucose (C₆H₁₂O₆) is added to 750 g of water.3.Calculate the molality of glucose in the solution.4.Mole Fraction Problem5.A solution is prepared by mixing 100 g of methyl alcohol (CH₃OH) and 100 g of water.6.Calculate the mole fraction of methyl alcohol.

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Study Guide2. Quiz: Concentration Units1. QuestionA homogeneous mixture of two or more substances is a(n)Answer Choices•Compound•Solution•ElementCorrect AnswerSolutionWhy This is CorrectAsolutionis a homogeneous mixture, which means it has thesame composition throughout. In asolution, the substances are evenly mixed, so you cannot see the different components separately.For ExampleSalt dissolved in water forms a solution.The salt spreads evenly through the water, making the mixture uniform.2.QuestionIn a solution, the host substance is theAnswer Choices•Solvent•Solute•MixtureCorrect AnswerSolvent

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Study GuideWhy This is CorrectThesolventis the substance that is present in a larger amount and dissolves the other substance(solute) to form a solution.For ExampleIn salt water,wateris the solvent andsaltis the solute.3. QuestionThe solute is theAnswer Choices•host substance•solid phase only•dissolved substanceCorrect Answerdissolved substanceWhy This Is CorrectAsoluteis the substance that getsdissolvedin another substance to form a solution.The substance that does the dissolving is called thesolvent, not the solute.For example:•In saltwater,saltis the solute because it dissolves.•Wateris the solvent because it dissolves the salt.So, the correct definition of a solute isthe dissolved substance.4. QuestionMeasuring the relative mass of the constituents of a solution is known as

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Study GuideAnswer Choices•relative masses•mass percents•common massCorrect Answermass percentsWhy This is CorrectMass percent is a concentration unit that expresses the relative mass of each component in asolution. It tells us how much of the solute is present compared to the total mass of the solution,multiplied by 100.For ExampleIf 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. QuestionA solution is made by mixing 250.0 g of hexane and 50.0 g of octanol. What is the mass percent ofthe octanol?Answer Choices•83.3%•16.7%•20.0%Correct Answer16.7%

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Study GuideWhy This is CorrectMass percent is calculated using the formula:Here, octanol is the solute.Total mass of solution = 250.0 g + 50.0 g = 300.0 gFor ExampleIf 10 g of salt is mixed with 90 g of water, the mass percent of salt is:(10/100) × 100 = 10%6.QuestionA solution of sodium chloride and water consists of3.0 moles of sodium chlorideand7.0 moles ofwater.What is themole fraction of sodium chloride?Answer Choices•0.43•0.70•0.30Correct Answer0.30Why This is CorrectThe mole fraction of a substance is calculated by dividing the number of moles of that substance bythetotal number of moles in the solution.

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Study GuideFor ExampleIf a class has 3 boys and 7 girls, the fraction of boys is3/10 = 0.307. QuestionThe number of moles of solute per kilogram of solvent is known asAnswer Choices•molarity•molality•solubilityCorrect AnswermolalityWhy This Is CorrectMolalityis defined as the number ofmoles 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 amolality of 1 m.Molality is especially useful because it doesnot change with temperature, making it reliable forscientific calculations.8. QuestionThe termaqueous solutionmeans the

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