[Raul-7]

I have this expermiental lab due in a couple of days and I'm having trouble determining the correct value of R in PV/nT=R . Can someone help me?

[Raul-7]

Ok, this is how I did it and got the wrong value. The value should be around 0.08206 L atm mol K.

I got both chemical equations:

2NaHCO3 ---> Na2CO3 + CO2 + H2O

Use the molar mass: 84.02g/mol to break down all the massess (on the chart) into mols. Then divided by 2 to get the mols of the product Na2CO3. That's how I got 'n'.

For the temp. I used the temps on the chart as Tfinal and the original 750C as Tinitial. Coverted them to Kelvin and substrated them to get the value of 'T'.

P, I just divided the Torr values by 760 to convert them into atm.

V, already given.

For example, for the first row I got. PV=(4.12)2=8.24 ; nT=0.0310(1073-1023)=1.55

R=8.24/1.55= 5.32

Am I missing something, is this value totally wrong or are these values plotted on a graph somehow supposed to give me the right value when I do the slope? 

Na2CO3 ---> Na2O + CO2

[billnotgatez]

http://www.chemicalforums.com/index.php?PHPSESSID=3392590becf500236fc17dc939d8c85c&topic=8610.msg39808

maybe that will help

[Donaldson Tan]

2NaHCO3 ---> Na2CO3 + CO2 + H2O

The underlined product should b Na₂O

2NaHCO₃ -> Na₂O + H₂O + 2CO₂

At 750C, even water is a gas.

For every 2 mole of Sodium Bicarbonate that breaks down, 3 moles of gas are produced.

Na2CO3 ---> Na2O + CO2

For every mole of sodium carbonate that breaks down, 1 mole of gas is produced.

[Raul-7]

2NaHCO3 ---> Na2CO3 + CO2 + H2O

The underlined product should b Na₂O

2NaHCO₃ -> Na₂O + H₂O + 2CO₂

At 750C, even water is a gas.

For every 2 mole of Sodium Bicarbonate that breaks down, 3 moles of gas are produced.

Na2CO3 ---> Na2O + CO2

For every mole of sodium carbonate that breaks down, 1 mole of gas is produced.

Thanks. So both reactions have Na2O as products? The 3 moles of gas is 2 for CO2 and 1 for H2O - correct?

For the n value, what product should I use to determine it. I mean I was thinking the Na2O but the formula is for a gas, so I assume for the first one I use CO2 or H2O? Or both? How can I combine both? 

[billnotgatez]

Why did you do this

(1073-1023)

Also are you really sure it is .0310 moles

[Donaldson Tan]

Thanks. So both reactions have Na2O as products?

Yes. This is specifically mentioned in the 3rd line of the question. (see screenshot of word doc)

The 3 moles of gas is 2 for CO2 and 1 for H2O - correct?

Yes. This is only applicable to 2 moles of Sodium Bicarbonate (NaHCO₃).

For the n value, what product should I use to determine it. I mean I was thinking the Na2O but the formula is for a gas, so I assume for the first one I use CO2 or H2O? Or both? How can I combine both? 

Assume both CO₂ and H₂O are perfect gas. For example, 8.41g of NaHCO₃ is being used. This is equivalent to 0.1 mole. Hence, 0.15 mole of gas (0.10 mol of CO₂ & 0.05 mol of H₂O) must be produced.

[Raul-7]

Thanks. So both reactions have Na2O as products?

Yes. This is specifically mentioned in the 3rd line of the question. (see screenshot of word doc)

The 3 moles of gas is 2 for CO2 and 1 for H2O - correct?

Yes. This is only applicable to 2 moles of Sodium Bicarbonate (NaHCO₃).

For the n value, what product should I use to determine it. I mean I was thinking the Na2O but the formula is for a gas, so I assume for the first one I use CO2 or H2O? Or both? How can I combine both? 

Assume both CO₂ and H₂O are perfect gas. For example, 8.41g of NaHCO₃ is being used. This is equivalent to 0.1 mole. Hence, 0.3 mole of gas (0.2 mol of CO₂ & 0.1 mol of H₂O) must be produced.

Ok, I got it. For example, in the first part. The first row, the # of moles for both is 0.093 since the ratio is 2:3 and there is 0.062 moles of solid, thus there is 0.093 moles of gas. As for the temperature, the difference in the first one is +50K, correct?

Thanks.

[Donaldson Tan]

For example, for the first row I got. PV=(4.12)2=8.24 ; nT=0.0310(1073-1023)=1.55

R=8.24/1.55= 5.32

You are told specifically to plot the graph of PV against nT. You will get the value R from the gradient of the line of best fit

Can you list the PV values with their corresponding nT?

I have already showed you how to calculate n, so should be able to evaluate the values of PV and nT corresponding to the data provided. Take note that T is in terms of Kelvin, not Celcius.

[Raul-7]

But why is your ratio 1:3 (in that example 0.1 to 0.3 moles gas)? It's 3 moles of gas for every 2 moles of NaHCO3? The ratio should be 2:3?


And is the T value for row #1 (800+273) - (750+273) ? Or just (800+273) ?



And for the graph, you mean I make one line of PV and one line of nT ?

[Donaldson Tan]

But why is your ratio 1:3 (in that example 0.1 to 0.3 moles gas)? It's 3 moles of gas for every 2 moles of NaHCO3? The ratio should be 2:3?T ?

Forgive my overlook. You are right. I have corrected the example in Reply #6

And is the T value for row #1 (800+273) - (750+273) ? Or just (800+273) ?

T = 800 + 273 = 1073K

And for the graph, you mean I make one line of PV and one line of nT ?

Plot 1 graph - the y-axis is PV and the x-axis is nT

[Raul-7]

Last question, how do I graph a line with 3-axes on Excel. It doesn't understand that I want PV vs nT as my axes, it only allows me two so I'm stuck with R vs nT.

[Donaldson Tan]

You are not required evaluate R directly from the data.

Just plot PV against nT. The gradient of this line corresponds to R.

You definitely don't know how to use Microsoft Excel.

If you use XY Scatter option in Chart Wizard, choose the subtype with lines joining the points together.

The XY Scatter option allows u to specify manually which column contains the x-coordinates and which column contains the y-coordinates.