Consider a 790-mL container at 21 °C in which the pressure of argon is 1.98x106 Pa.

a) What mass of argon (in g) is present in this container?
b) How many molecules is this?

a) Use PV = nRT, solve for n

n = grams/molar mass, solve for grams.

b) 1 mole contains 6.022 x 10^23 molecules. How many moles do you have?

WHich (R) do I have to use

To find the mass of argon present in the container, you can use the ideal gas law equation:

PV = nRT

P = pressure of the gas (in Pa)
V = volume of the container (in m^3)
n = number of moles of gas
R = ideal gas constant (8.314 J/(mol·K))
T = temperature of the gas (in Kelvin)

First, let's convert the temperature from Celsius to Kelvin:

T = 21 °C + 273.15 = 294.15 K

Next, let's convert the volume of the container from milliliters to cubic meters:

V = 790 mL = 790/1000 = 0.79 L = 0.79/1000 = 0.00079 m^3

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

n = PV / RT

Substituting the given values:

n = (1.98x10^6 Pa) * (0.00079 m^3) / ((8.314 J/(mol·K)) * (294.15 K))

Calculating this, we find:

n ≈ 0.094 moles

To find the mass of argon, we need to use the molar mass of argon which is approximately 39.95 g/mol. We can calculate the mass using the formula:

Mass (g) = number of moles of gas * molar mass of gas

Substituting the values:

Mass (g) = 0.094 moles * 39.95 g/mol

Calculating this, we find:

Mass (g) ≈ 3.76 g

Therefore, the mass of argon present in the container is approximately 3.76 grams.

Now, to find the number of molecules of argon, we can use Avogadro's number (6.022x10^23 molecules/mol). Multiplying the number of moles by Avogadro's number will give us the number of molecules:

Number of molecules = number of moles * Avogadro's number

Substituting the values:

Number of molecules ≈ 0.094 moles * (6.022x10^23 molecules/mol)

Calculating this, we find:

Number of molecules ≈ 5.659 x 10^22 molecules

Therefore, the number of molecules of argon present in the container is approximately 5.659 x 10^22 molecules.