The following reaction is carried out as a source of hydrogen gas in the laboratory:

Zn(s) + 2HCl(aq) ---> ZnCl2(aq) + H2(g)
If 305 mL of hydrogen gas is collected over water at 25 °C at a total pressure of 746 mm Hg , how many grams of Zn reacted?

Use PV = nRT to solve for n = number of moles H2 gas. Remember V is in liters, p in atmospheres, and T in Kelvin. You will need to look up the vapor pressure of water at 25 C and calculate pressure as follows:

(746 mm - mm vapor pressure @ 25C)/760.
Using the coefficients in the balanced equation, convert moles H2 to mols Zn.
Finally, convert moles Zn to grams. g = moles x molar mass.

To determine the grams of Zn reacted, we need to use the ideal gas law equation:

PV = nRT

Where:
P = pressure of the gas
V = volume of the gas
n = number of moles of the gas
R = ideal gas constant
T = temperature in Kelvin

First, let's convert the volume of the hydrogen gas collected over water from milliliters (mL) to liters (L):
305 mL = 305/1000 = 0.305 L

The equation is balanced, so the stoichiometric ratio tells us that for every 1 mole of Zn, we get 1 mole of H2 gas.

Next, we need to convert the total pressure from mm Hg to atmospheres (atm):
746 mm Hg = 746/760 atm (since 1 atm = 760 mm Hg)

Now, we can rearrange the ideal gas law equation to solve for the number of moles (n) of H2 gas:
n = PV / RT

We need to convert the temperature from Celsius to Kelvin:
25 °C + 273.15 = 298.15 K

The ideal gas constant (R) is 0.0821 L*atm/(mol*K).

Now, we can substitute the values into the equation:
n = (0.305 L * 746/760 atm) / (0.0821 L*atm/(mol*K) * 298.15 K)
n ≈ 0.0127 mol

Since the stoichiometric ratio between Zn and H2 is 1:1, the number of moles of H2 gas (0.0127 mol) is also equal to the number of moles of Zn reacted.

To convert moles of Zn to grams, we need to use the molar mass of Zn, which is 65.38 g/mol.

Mass of Zn = moles of Zn * molar mass of Zn
Mass of Zn = 0.0127 mol * 65.38 g/mol
Mass of Zn ≈ 0.8296 g

Therefore, approximately 0.8296 grams of Zn reacted in the given reaction.