**Calculating the Number of Molecules in Ammonia Gas**

To calculate the number of molecules present in a given volume of ammonia gas, we need to use the ideal gas law equation:

PV = nRT

Where:
P = Pressure (in atm)
V = Volume (in liters)
n = Number of moles
R = Gas constant (0.0821 L.atm/mol.K)
T = Temperature (in Kelvin)

We have the following information:

Pressure (P) = 2 atm
Volume (V) = 350 cm³ = 0.35 L
Temperature (T) = 273 K

**Step 1: Convert volume to liters**
We convert the given volume of ammonia gas from cubic centimeters to liters, as the ideal gas law requires the volume to be in liters.

1 cm³ = 1 mL
1 L = 1000 mL

Therefore, 350 cm³ = 350 mL = 0.35 L

**Step 2: Convert pressure to atm**
The pressure is already given in atm, so no conversion is required.

**Step 3: Convert temperature to Kelvin**
The temperature is given in degrees Celsius, so we need to convert it to Kelvin by adding 273.

Temperature in Kelvin = 273 K

**Step 4: Substitute the values into the ideal gas law equation**

PV = nRT

(2 atm) x (0.35 L) = n x (0.0821 L.atm/mol.K) x (273 K)

0.7 atm.L = n x 22.4148 L.atm/mol

**Step 5: Solve for n (number of moles)**

n = (0.7 atm.L) / (22.4148 L.atm/mol)

n ≈ 0.0312 mol

**Step 6: Convert moles to number of molecules**
To convert moles to number of molecules, we use Avogadro's number (6.022 x 10^23 molecules/mol).

Number of molecules = n x Avogadro's number

Number of molecules = 0.0312 mol x (6.022 x 10^23 molecules/mol)

Number of molecules ≈ 1.877 x 10^22 molecules

Therefore, there are approximately 1.877 x 10^22 molecules present in 350 cubic cm of ammonia gas at 273 K and 2 atm pressure.

1.8801 * 10^22