Teacher Notes
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Teacher Notes![]() Molar Volume of HydrogenStudent Laboratory KitMaterials Included In Kit
Copper wire, Cu, 18-gauge, 1.8 m
Hydrochloric acid, HCl, 2 M, 200 mL Magnesium ribbon, Mg, 45 cm Metric rulers, 15 One-hole rubber stoppers, size 00, 15 Additional Materials Required
Water, distilled or deionized
Barometer (one per classroom) Beaker, 400-mL Graduated cylinder, 10-mL, tall-form Ice water (optional) Pencil Scissors or wire cutters Steel wool or sandpaper (optional) Thermometer Wash bottle or plastic pipet Safety PrecautionsHydrochloric acid is a corrosive liquid. Avoid contact with eyes and skin and clean up all spills immediately. Magnesium metal is a flammable solid. Keep away from flames and other sources of ignition. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information. DisposalPlease consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The water bath solutions remaining after the hydrogen gas has been collected will be acidic. These should be neutralized by adding base (sodium bicarbonate is a good choice) and then flushed down the drain with excess water according to Flinn Suggested Disposal Method #24b. Lab Hints
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Correlation to Next Generation Science Standards (NGSS)†Science & Engineering PracticesPlanning and carrying out investigationsAnalyzing and interpreting data Using mathematics and computational thinking Disciplinary Core IdeasMS-PS1.A: Structure and Properties of MatterMS-PS1.B: Chemical Reactions HS-PS1.A: Structure and Properties of Matter HS-PS1.B: Chemical Reactions Crosscutting ConceptsScale, proportion, and quantityStability and change Performance ExpectationsMS-PS1-1: Develop models to describe the atomic composition of simple molecules and extended structures. Answers to Prelab QuestionsReaction of 0.028 g of magnesium with excess hydrochloric acid generated 31.0 mL of hydrogen gas. The gas was collected by water displacement in a water bath at 22 °C. The barometric pressure in the lab that day was 746 mm Hg.
Sample Data{13961_Data_Table_2}
*The conversion factor for the magnesium used in this experiment was 0.0094 g/cm. {13961_Data_Table_3}
Answers to QuestionsSee the results table in Sample Data for the results of the calculations (Questions 1–5).
ReferencesThis activity was adapted from Flinn ChemTopic™ Labs, Volume 9, The Gas Laws; Cesa, I., Editor; Flinn Scientific: Batavia IL, 2003. Recommended Products
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Student Pages
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Student Pages![]() Molar Volume of HydrogenIntroductionAirbags have been required safety features on new cars since the 1980s and are credited with saving thousands of lives over that time. Airbags contain a compound that decomposes to release nitrogen gas upon impact from a collision. How much gas must be generated to fill an airbag? The amount of gas needed to fill any size container can be calculated if we know the molar volume of the gas. Concepts
BackgroundAvogadro’s law states that equal volumes of gases contain equal numbers of molecules under the same conditions of temperature and pressure. It follows, therefore, that all gas samples containing the same number of molecules will occupy the same volume if the temperature and pressure are kept constant. The volume occupied by one mole of a gas is called the molar volume. In this experiment, the molar volume of hydrogen gas will be determined at standard temperature and pressure (STP, equal to 273 K and 1 atm). {13961_Background_Equation_1}
If the reaction is carried out with excess hydrochloric acid, the volume of hydrogen gas obtained will depend on the number of moles of magnesium as well as on the pressure and temperature. The molar volume of hydrogen can be calculated if we measure the volume occupied by a sample containing a known number of moles of hydrogen gas. Since the volume will be measured under laboratory conditions of temperature and pressure, the measured volume must be corrected to STP conditions before calculating the molar volume. The relationship among the four gas variables—pressure (P), volume (V), temperature (T) and the number of moles (n)—is expressed in the ideal gas law (Equation 2), where R is a constant called the universal gas constant. {13961_Background_Equation_2}
The ideal gas law reduces to Equation 3, the combined gas law, if the number of moles of gas is constant. The combined gas law can be used to calculate the volume (V2) of a gas at STP (T2 and P2) from the volume (V1) measured under any other set of laboratory conditions (T1 and P1). In using either the ideal gas law or the combined gas law, remember that temperature must be always be expressed in units of kelvins (K) on the absolute temperature scale.
{13961_Background_Equation_3}
In this activity, hydrogen gas will be collected by the displacement of water in an inverted graduated cylinder using the apparatus shown in Figure 1.
{13961_Background_Figure_1}
The total pressure of the gas in the cylinder will be equal to the barometric (air) pressure. However, the gas in the cylinder will not be pure hydrogen. The gas will also contain water vapor due to the evaporation of the water molecules over which it is being collected. According to Dalton’s law, the total pressure of the gas is equal to the partial pressure of hydrogen plus the partial pressure of water vapor (Equation 4).
{13961_Background_Equation_4}
The vapor pressure of water depends only on the temperature (see Table 1). Table 1. Vapor Pressure of Water at Different Temperatures {13961_Background_Table_1}
Experiment OverviewThe purpose of this experiment is to determine the volume of one mole of hydrogen gas at standard temperature and pressure (STP). Hydrogen will be generated by the reaction of a known mass of magnesium with excess hydrochloric acid in an inverted graduated cylinder filled with water. The volume of hydrogen collected by water displacement will be measured and corrected for differences in temperature and pressure in order to calculate the molar volume of hydrogen at STP. Materials
Copper wire, Cu, 18-gauge, 10-cm long
Hydrochloric acid, HCl, 2 M, 10 mL Magnesium ribbon, Mg, 1-cm pieces, 2 Water, distilled or deionized Barometer Beaker, 400-mL Graduated cylinder, 10-mL Metric ruler One-hole rubber stopper, size 00 Pencil Scissors or wire cutters Thermometer Wash bottle or plastic pipet Prelab QuestionsReaction of 0.028 g of magnesium with excess hydrochloric acid generated 31.0 mL of hydrogen gas. The gas was collected by water displacement in a water bath at 22 °C. The barometric pressure in the lab that day was 746 mm Hg.
Safety PrecautionsHydrochloric acid is a corrosive liquid. Avoid contact with eyes and skin and clean up all spills immediately. Magnesium metal is a flammable solid. Keep away from flames and other sources of ignition. Wear chemical splash goggles and chemical-resistant gloves and apron. Wash hands thoroughly with soap and water before leaving the laboratory. Procedure
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