If you have constant volume of gas how can you determine the number of moles it has? what equation do you use if your not given the constant volume value?

To determine the number of moles of gas when you have a constant volume, you can use the ideal gas law equation. The ideal gas law is given by the equation: PV = nRT.

In this equation:
P represents the pressure of the gas in units of pressure (such as atmospheres, pascals, etc.),
V represents the volume of the gas in units of volume (such as liters, cubic meters, etc.),
n represents the number of moles of gas,
R is the ideal gas constant (with a value of 0.0821 L·atm/(mol·K) or 8.314 J/(mol·K) depending on the unit of pressure and volume you use),
T represents the temperature of the gas in units of temperature (such as Kelvin).

If you have constant volume, the equation simplifies to: P = nRT/V.

If you are not given the constant volume value, you would need to measure or determine the volume of the gas under the conditions you are working with. Without the volume value, you won't be able to directly find the number of moles of gas.