how do I calculate pressure exerted by H2 in a Mg and HCl reaction? (weighing by redox)

Write the balanced chemical equation.

Mg + 2HCl ==> H2 + MgCl2

Do you have a mass for Mg? If so, convert mass Mg to mols, convert mols Mg to mols H2 (it's a 1:1 reaction for Mg to H2), then convert mols H2 to volume knowing that at STP a mol of H2 gas occupies 22.4 L. If I have guess wrong about having a mass of Mg, please give more details about the problem.

when I converted g to mols for Mg, I got 8.640, but it doesn't look right. original mass for Mg = .021g

It isn't. mols = g/atomic mass.

mols = 0.021/24.3 = ?? but you need to look up the atomic mass of Mg. Sometimes I don't remember that well.

To calculate the pressure exerted by H2 in a reaction between Mg and HCl, you need to consider the stoichiometry of the reaction and use the ideal gas law.

Here's a step-by-step guide on how to calculate the pressure:

1. Start by writing the balanced chemical equation for the reaction between Mg and HCl:
Mg + 2HCl -> MgCl2 + H2

2. Determine the moles of magnesium (Mg) used in the reaction. You can do this by dividing the given mass of Mg by its molar mass. For example, if you have 0.5 grams of Mg:
moles of Mg = (0.5 g Mg) / (24.31 g/mol Mg)

3. Use the coefficients in the balanced equation to find the moles of H2 produced. In this case, the stoichiometry is 1:1, so the moles of H2 will be the same as the moles of Mg.

4. Apply the ideal gas law, which states: PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the ideal gas constant (0.0821 L·atm/(mol·K)), and T is the temperature in Kelvin.

5. Rearrange the ideal gas law equation to solve for P:
P = (n * R * T) / V

6. Determine the volume (V) of the gas. This could be given in the problem statement or calculated based on the conditions provided.

7. Convert the temperature to Kelvin if it is given in Celsius. Simply add 273.15 to the Celsius temperature.

8. Plug in the values for n (moles of H2), R (ideal gas constant), T (temperature in Kelvin), and V (volume) into the equation from step 5.

9. Calculate the pressure (P) exerted by the H2 gas.

Remember to use consistent units throughout the calculation to obtain accurate results.