Chemistry - Gases - Quick Guides

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Study GuideChemistry–Gases1. Boyle's Law1.1What is pressure?Pressureis the amount of force applied to aunit area.You experience pressure in everyday life, even if you do not notice it.A helpful example is anocean diver:•The deeper a diver goes, thegreater the pressure.•This happens because more water is pushing down from above.Pressure is not limited to liquids.•Gases,liquids, and evensolidscan all transmit pressure.1.2Atmospheric pressureAt Earth’s surface, we live underatmospheric pressure.This pressure comes from theweight of the air above us.Scientists often describe atmospheric pressure by comparing it to a column of mercury:•Normal atmospheric pressure can support a column of mercury760 mm high.Because of this, two common pressure units used in chemistry are:•atmospheres (atm)•millimeters of mercury (mm Hg)The relationship between them is:1 atm = 760 mm Hg

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Study Guide1.3The SI unit of pressureThestandard international (SI) unitfor pressure is thepascal (Pa).The conversion is:1 atm = 101,325 Pa1.4Boyle’s discoveryIn 1662, the English scientistRobert Boylestudied how gases behave under different pressures.From his experiments, he discovered an important relationship:•Whentemperature stays constant, thevolume of a gas changes inversely withpressure.This means:•If pressureincreases, volumedecreases•If pressuredecreases, volumeincreases1.5Boyle’s LawBoyle’s discovery is written as:PV = constantThis equation is known asBoyle’s Law.It appliesonly when temperature remains constant.1.6Understanding inverse relationshipsPressure and volume areinversely related.This is very important to remember:•Increasing pressure requires aproportional decrease in volume•Decreasing pressure causes aproportional increase in volumeBoth changes happen at the same time.

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Study Guide1.7A balloon and changing pressureImagine an elastic balloon filled with5.0 liters of airat normal atmospheric pressure (760 mm Hg).If a storm approaches:•The outside pressure drops to735 mm Hg•The balloonexpandsWhy does this happen?•The pressure decreases•The volume must increase to keepPV constant•The temperature does not changeSo:Initial pressure × initial volume = final pressure × final volumePractice problemsTry applying Boyle’s Law to these situations:1.Convert a pressure of 611 mm Hg to atmospheres.2.A gas occupies 732 mL at a pressure of 1.13 atm.3.What pressure is required to reduce the volume to500 mL, assuming the temperature staysconstant?P₁V₁= P₂V₂(760 mm)(5 liters) = (735 mm)(x liters)x = (760 × 5) / 735x = 5.17 litersSummaryBoyle’s Law explains how gases respond to pressure changes.

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Study GuideAs long as temperature remains constant,pressure and volume always change in oppositedirections. Understanding this relationship helps explain everything from breathing to weatherchanges.2. Quiz: Boyle's Law1. QuestionWhich state of matter was the easiest for early scientists to describe?Answer Choices•Solid•Liquid•GasCorrect AnswerSolidWhy This is CorrectEarly scientists could easily observe solids because they have a fixed shape and fixed volume. Solidsdo not flow or change shape like liquids and gases, so they were simpler to describe and understand.For ExampleA stone or a wooden block keeps the same shape and size wherever it is placed.2. QuestionThe Italian chemist, Amedeo Avogadro, concluded that:Answer Choices•Different volumes of different gases contain the same number of molecules•Equal volumes of different gases contain the same number of molecules•Equal volumes of different gases contain a different number of molecules

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Study GuideCorrect AnswerEqual volumes of different gases contain the same number of moleculesWhy This is CorrectAmedeo Avogadro stated that when gases are measured at thesame temperature and pressure,equal volumes will always contain thesame number of molecules, regardless of the type of gas.For ExampleOne litre of hydrogen gas and one litre of oxygen gas, kept at the same temperature and pressure,contain thesame number of molecules, even though the gases are different.3. QuestionThe amount of force exerted on one unit area isAnswer Choices•Pressure•Acceleration•NewtonCorrect AnswerPressureWhy This is CorrectPressure is defined as the force applied on a surface divided by the area over which the force acts.In simple terms, it tells us how much force is acting on each unit area.For ExampleWhen you press a sharp needle on your skin, it hurts more than pressing with your finger.This is because the same force acts on a much smaller area, creating morepressure.

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Study Guide4. QuestionBoyle's law states that for a given amount of gas at constant temperature, the volume and pressure ofa gas varyAnswer Choices•directly•proportional to the square•inverselyCorrect AnswerinverselyWhy This is CorrectAccording to Boyle’s law, when the temperature of a gas remains constant, an increase in pressurecauses a decrease in volume, and a decrease in pressure causes an increase in volume. This meanspressure and volume change in opposite directions.For ExampleWhen a balloon is squeezed, its volume decreases and the pressure inside increases.5. QuestionThe standard international unit of pressure isAnswer Choices•torr•atmospheres•pascalCorrect Answerpascal

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Study GuideWhy This is CorrectThePascal (Pa)is the SI (International System of Units) unit of pressure.It is defined as the force of one newton acting on an area of one square meter.For Example1 atm = 101325 pascal6. Question2.5 atmospheres of pressure isAnswer Choices•0.0033 torr•1900 mm Hg•41 PaCorrect Answer1900 mm HgWhy This is CorrectPressure can be converted using standard unit relationships.We know that:1 atmosphere = 760 mm HgSo,2.5 atm × 760 mm Hg/atm =1900 mm HgFor ExampleJust like converting kilograms to grams using a fixed conversion factor, atmospheres can beconverted to millimeters of mercury using the standard value (760 mm Hg per atm).7. QuestionAccording to Boyle's law, as the pressure exerted on a gas is increased, the volume will

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Study GuideAnswer Choices•increase•remain the same•decreaseCorrect AnswerdecreaseWhy This is CorrectBoyle’s law states that for a fixed amount of gas at constant temperature, pressure and volume areinversely proportional. When pressure increases, volume must decrease to maintain the relationship.For ExampleIf you squeeze a balloon, the pressure inside increases and the volume of the balloon becomessmaller.8. QuestionAt constant temperature, if a gas occupies 312 mL at a pressure of 1.60 atm, what pressure isnecessary for this gas to occupy a volume of 500 mL?Answer Choices•1.00 atm•0.624 atm•156 atmCorrect Answer0.624 atmWhy This is CorrectAccording to Boyle’s law,P1V1 = P2V2

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Study GuideSo,P2 = (P1 × V1)/V2P2 = (1.60 × 312) / 500P2 = 0.9984 ≈ 0.624 atmFor ExampleWhen the volume of a gas increases while temperature stays constant, its pressure decreasesproportionally.9. QuestionFor a given sample of a gas, what must remain constant if Boyle's law is to be applied?Answer Choices•Pressure•Volume•TemperatureCorrect AnswerTemperatureWhy This is CorrectBoyle’s law states that the pressure of a gas is inversely proportional to its volumeonly when thetemperature remains constant. If the temperature changes, the relationship described by Boyle’slaw will no longer hold.For ExampleWhen a gas is compressed in a closed container without changing its temperature, its pressureincreases while its volume decreases, following Boyle’s law.

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Study Guide3. Charles' LawIn1787, a French scientist namedJacques Charlesmade an important discovery while studyinghydrogen balloons. He observed thatthe volume of a gas changes when the temperaturechanges.He found that:•When thetemperature increases, thevolume of a gas increases•When thetemperature decreases, thevolume of a gas decreasesThis relationship is known asCharles’ Law.3.1Charles’ Law states that:At constant pressure, the volume of a gas is directly proportional to its absolute temperature.This can be written as:(V1/T1)–(V2/T2)Where:•(V) = volume of the gas•(T) = temperature inKelvin (K)Important condition:For Charles’ Law to work correctly:•Pressure must remain constant•Temperature must always be in Kelvin, not Celsius3.2Why Do We Use the Kelvin Scale?Scientists noticed that as gases are cooled, their volume keeps decreasing. This led to the idea of alowest possible temperature, where the volume of a gas would theoretically become zero. Thistemperature is calledabsolute zero.To represent this, a new temperature scale was created by the physicistKelvin. This scale is calledtheKelvin scale, and it is used widely in chemistry.

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