Gas Stoichiometry Lab

CHS

Richard Jackson

2007

Data Table:

__ Mg + __ HCl __ + __ 1. Length of Magnesium ______ cm2. Mass of Magnesium ______ g3. Moles of Hydrogen Gas ______ mol (n)4. Actual Yield of H2 ______ mL5. Temperature of water ______ °C6. Temperature in Kelvin ______ K7. Air Pressure of Room ______ mmHg8. Vapor Pressure of Water ______ mmHg9. Partial Pressure of H2 ______ mmHg10. Partial Pressure of H2 ______ atm11. Theoretical Yield of H2 ______

mL12. Percent Yield of H2 ______ %

Complete the Single Replacement / Displacement Reaction below and Balance it

• __ Mg + __ HCl __ + __

Calculate Necessary Mg

• Determine length (cm) of Mg needed to produce 25-30mL of hydrogen gas. Determine to 4 decimal places.

• Use the balanced equation• Use the 1 mol gas = 22.414 L gas (@STP)• Use Mg 24.31g/mol• Use 1cm Mg = 0.0089 grams (this value may

vary based on amount of oxidation - )

Cut piece of magnesium metal.

Get mass of Mg

• Use the analytical balance in stock room to get the mass of the metal.

Roll up magnesium strip into a little ball.

Wrap up magnesium strip in a copper wire cage leaving a 2 inch hook.

Mg and Cu

• Why is a copper cage used? • We are placing the contraption into HCl –

what will / won’t happen?

Activity Series

Add 8 mL of 6 M Hydrochloric Acid into a gas collection tube.

Slowly add distilled water, stacking it on top of the acid.

Why is acid always added to water, and not the reverse?

• A large amount of heat is released when strong acids are mixed with water. Adding more acid releases more heat. If you add water to acid, you form an extremely concentrated solution of acid initially. So much heat is released that the solution may boil very violently, splashing concentrated acid out of the container! If you add acid to water, the solution that forms is very dilute and the small amount of heat released is not enough to vaporize and spatter it.

• So Always Add Acid to water, and never the reverse

• BE CAREFUL – We are doing the opposite (in tall glassware – so the risk is greatly reduced).

Insert magnesium/copper wire cage.

Make sure the hook catches

Insert the one hole stopper

Cover stopper hole with gloved finger and invert the tube.

Put the tube into the water before removing your finger.

Connect tube to buret clamp. Watch the acid swirl through the water and down the tube.

Look- Bubbles!

Bubbles

Video Clip of Bubbles

The temperature = ? °C

30°C

20°C

?°C

Convert °C into K

• K = °C + 273

Measuring the Volume of Hydrogen Gas

Water level

Make the water levels equalThe volume is ? mL

Actual Yield ? mL

Remove the stopper and dump the remaining water.

Pressure

• Get today’s barometric pressure.• Most stations have it in inches Hg

– Convert to mm Hg– 1 inch = 2.54cm

690 mmHg

680 mmHg

Room Air Pressure? mmHg

Dalton’s Law• The pressure above our aqueous solution is not

all hydrogen gas. There is also water vapor – it is a mixture.

• Dalton’s Law states that the total pressure of a mixture (PT) is equal to the pressure of the individual gases. PT = PH2 + PH2O

• PT is our atmospheric pressure here at CHS (Hill Field data used).

• PH2O , water-vapor pressure, is gathered from the table based on temperature.

Temperature - Vapor Pressure Chart

Calculate Partial Pressure of H2

• PT = PH2O + PH2

• PT – PH2O = PH2

Convert Partial Pressure of H2 from mmHg into atm

• ____ mmHg ( 1 atm ) =

( 760 mmHg)

Convert mass of Mg into Moles Mg

• ____ g Mg ( 1 mole Mg ) =

( 24.31 g Mg )

Convert Moles of Mg into Moles H2

• ____ mol Mg ( ? mol H2 ) =

( ? mol Mg )

Calculate the Theoretical Yield of H2 gas in mL

• PV = nRT

• VH2 = nRT / P

• VH2 = ( mol)(0.0821 L atm) ( K)(1000 mL)

( atm ) mol K ( 1 L )

• VH2 = mL

Calculate the Percent Yield of H2

• Percent Yield = (Actual Lab Yield) x 100 = (Theoretical Yield)

• Percent Yield = ( ) x 100 = ( )

• Percent Yield = %

The End