# Calculate Mass of Non-Volatile Solute in Octane to Reduce V.P. 80%

• Vriska
In summary, the conversation discusses how to calculate the mass of a non-volatile solute with a molar mass of 40 that needs to be dissolved in 114 g of octane to reduce its vapor pressure to 80%. The formula v.p = v.p_pure * mole fraction of solvent in solution is used to calculate the mass, which is determined to be 10 grams. There is some confusion about the total number of moles in the solution, but it is clarified that there is 1 mole of octane and 0.25 moles of solute is needed to achieve a solvent mole fraction of 0.8. It is also mentioned that the amount of solvent in the gas phase is negligible and does not
Vriska
Thread moved from the technical forums, so no Homework Template is shown
I've got this, calculate the mass of a non volatile solute (molar mass 40) that should be dissolved in 114 g of octane to reduce its vapour pressure to 80 percent

I do this, v.p = v.p_pure * mole fraction of solvent in solution

we have 1 mole of solvent and let's take n moles of substance so .8 = 1/(n+1) doing all this we'll get our mass to be 10 grams

is this right? some people are getting 8.

annd one more thing, there's isn't actually 1 mole in the solution is there? some would exist in the gas state, say p =1, how would I theoretically get the exact number of moles I've got to add to this? Do I need to know the volume and temperature too?

Your answer is right. I think some people are assuming there is 1 mol total solution, so 0.8 mol octane and 0.2 mol solute. But there is 1 mol octane, so you need 0.25 mol solute to get a solvent mole fraction of 0.8.
You're meant to assume that the amount of solvent in the gas phase is negligible compared to the liquid. If this were not true, you would indeed have to make corrections for the gas phase fraction.

jim mcnamara

## 1. What is the purpose of calculating the mass of non-volatile solute in octane to reduce vapor pressure (V.P.) of 80%?

The purpose of this calculation is to determine the amount of non-volatile solute that needs to be added to octane in order to reduce its vapor pressure by 80%. This can be useful in various industrial and scientific applications, such as controlling the evaporation rate of a substance or adjusting its boiling point.

## 2. How is the mass of non-volatile solute in octane calculated?

The mass of non-volatile solute in octane can be calculated using the equation: Mass of solute = (V.P. of pure octane - V.P. of mixture) / (V.P. of pure octane) * mass of octane. This equation takes into account the vapor pressures of pure octane and the mixture, as well as the mass of octane used.

## 3. What is a non-volatile solute?

A non-volatile solute is a substance that does not easily evaporate at room temperature. This means that it has a low vapor pressure and will not contribute significantly to the overall vapor pressure of a mixture.

## 4. What is octane?

Octane is a hydrocarbon compound that is commonly used as a fuel in internal combustion engines. It is a highly volatile substance with a low boiling point, which makes it useful for producing energy through combustion.

## 5. Why is it important to reduce the vapor pressure of octane?

Reducing the vapor pressure of octane can have several benefits, such as controlling the rate of evaporation and reducing the risk of fire or explosion. In addition, it can also help improve the performance and efficiency of engines by preventing fuel loss and improving the combustion process.

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