The hydrogen gas generated when calcium metal reacts with water is collected over water at 20 degrees c. the volume of the gas is 641 ml and the pressure is 988mmhg. what is the mass in grams of the hydrogen gas obtained? the vapor pressure of water at 20 degrees c is 17.54 mmhg.

The mass of hydrogen gas obtained is 0.068 g of hydrogen gas.

The equation of the reaction is;

Ca(s) +2H2O(l) →Ca(OH)2(aq) + H2(g)

We have to obtain the number of moles of hydrogen gas produced using the information in the question.

P =  988mmHg - 17.54 mmHg = 1.28 atm

V = 641 mL or 0.641 L

T = 20 + 273 = 293 K

n = ?

R = 0.082 atmLK-1mol-1

From;

PV = nRT

n = PV/RT

n = 1.28 atm × 0.641 L/ 0.082 atmLK-1mol-1 × 293 K

n = 0.82/24.026

n = 0.034 moles

Mass of hydrogen =  0.034 moles × 2 g/mol = 0.068 g of hydrogen gas

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There is 0.0677 grams of H2 gas obtained

Explanation:

Step 1: Data given

The total pressure (988 mmHg) is the sum of the pressure of the collected hydrogen + the vapor pressure of water (17.54 mmHg).  

ptotal = p(H2)+ p(H2O)

p(H2) = ptotal - pH2O = 988 mmHg - 17.54 mmHg = 970.46 mmHg

Step 2: Calculate moles of H2 gas

Use the ideal gas law to calculate the moles of H2 gas

PV = nRT

n = PV / RT

 ⇒ with p = pressure of H2 in atm = 970.46 mmHg * (1 atm /760 mmHg) = 1.277 atm

⇒ V = volume of H2 in L = 641 mL x (1 L / 1000 mL) = 0.641 L

⇒ n = the number of moles of H2 = TO BE DETERMINED

⇒ R = the gas constant = 0.08206 L*atm/K*mol

⇒ T = the temperature = 20.0 °C = 293.15 Kelvin

n = (1.277)(0.641) / (0.08206)(298.15) = 0.0335 moles H2

Step 3: Calculate mass of H2

Mass of H2 = moles H2 ¨molar H2

0.0335 moles H2 * 2.02 g/mol H2  = 0.0677g H2

There is 0.0677 grams of H2 gas obtained

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