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# volume in chemistry watch

1. 10.0g of calcium nitrate is heated to 100kPa and a temperature of 300 c and it fully decomposes as shown in the equation below.

2Ca(NO3)2 → 2CaO + 4NO2 + O2

calculate
a) the volume of nitrogen dioxide evolved
b) the volume of oxygen evolved
c) the total volume of gas evolved
2. Use the formula: PV = nRT

Where:

P = Pressure (Pa)
V = Volume (m^3)
n = number of moles
R = Gas constant = 8.31 kJ/Mol
T = Temperature in Kelvin. To convert degrees Celsius to kelvin (K) add 273 to whatever the degrees c is.

Also, you've been given the mass of calcium nitrate, this is your reactant, you can use this mass to work out the moles of calcium nitrate (using the formula: moles = mass/Mr). From the moles of calcium nitratre you can use the ratios of moles given from the equation to work out the moles of nitrogen dioxide and oxygen. Then, you simply just sub in your values into PV = nRT (after rearranging for V) and get an answer.

If you need any more guidance, don't hesitate to ask!
3. (Original post by Kozmo)
Use the formula: PV = nRT

Where:

P = Pressure (Pa)
V = Volume (m^3)
n = number of moles
R = Gas constant = 8.31 kJ/Mol
T = Temperature in Kelvin. To convert degrees Celsius to kelvin (K) add 273 to whatever the degrees c is.

Also, you've been given the mass of calcium nitrate, this is your reactant, you can use this mass to work out the moles of calcium nitrate (using the formula: moles = mass/Mr). From the moles of calcium nitratre you can use the ratios of moles given from the equation to work out the moles of nitrogen dioxide and oxygen. Then, you simply just sub in your values into PV = nRT (after rearranging for V) and get an answer.

If you need any more guidance, don't hesitate to ask!

you know in the bold bit, how do i do that? it sounds proper silly but I'm confused. do i times by 4 because its 4NO2?
4. (Original post by tomlfc)
you know in the bold bit, how do i do that? it sounds proper silly but I'm confused. do i times by 4 because its 4NO2?
No, it's not silly, it's really easy to be confused! Well, we know the moles of the calcium nitrate and we know in the equation it has the molar number of '2', this is the large 2 in front of the calcium nitrate molecule. We compare this number to any other molecule (compound) in the products in question.

For example, nitrogen dioxide is 4NO2, as a result the ratio is 2:4, which is just 1:2. Because of this we multiply the moles of the calcium nitrate by two (due to the ratio) to get the moles of the nitrogen dioxide. We can then sub this, along with the other values, into our rearranged formula and get the volume!
5. (Original post by Kozmo)
No, it's not silly, it's really easy to be confused! Well, we know the moles of the calcium nitrate and we know in the equation it has the molar number of '2', this is the large 2 in front of the calcium nitrate molecule. We compare this number to any other molecule (compound) in the products in question.

For example, nitrogen dioxide is 4NO2, as a result the ratio is 2:4, which is just 1:2. Because of this we multiply the moles of the calcium nitrate by two (due to the ratio) to get the moles of the nitrogen dioxide. We can then sub this, along with the other values, into our rearranged formula and get the volume!
im not sure I've done this right, i got 175 for NO2 and 350 for O2 as their volumes
6. (Original post by tomlfc)
im not sure I've done this right, i got 175 for NO2 and 350 for O2 as their volumes
I don't think this is correct. When you rearrange the formula for V you should get:

V = nRT/P

The moles of Calcium Nitrate = 10/164.1 = 100/1641 mol - I'm going to leave it as a fraction for accuracy.

We know Calcium nitrate : nitrogen dioxide is 1:2, so we multiply this by two to get the moles of nitrogen dioxide = 200/1641.

Similarly, we know the ratio of calcium nitrate to oxygen = 2:1, so we divide it by two to get the moles of oxygen. = 50/1641.

We simply substitute these values into the formula to get the volume.
Have a go at this.

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Updated: October 18, 2015
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