[Ultrametric]

Hi all,

I'm currently stuck on a Gas question:

Determine the GroupIIA metal cation in a metal carbonate that decomposed into the metal oxide and carbon dioxide. 0.211 grams of the metal carbonate decomposed in a 285mL vessel, Carbon dioxide was collected over water at 25.00C and exerted a pressure of 93.6mmHg.

How can I identify this metal? I tried calculating the density of the gas; I got 0.123 atm, but I'm not sure, based on the wording of the question, if I am dealing with partial pressure or total. I can't seem to get an answer if I assume it is CO2partial, which is how the question is phrased. I tried mole fractions, but I don't have Ntotal.

Please H_elp Final tomorrow!  

[Borek]

How many moles of gas? How many moles of carbonate? What is molar mass of the metal?

[Ultrametric]

How many moles of gas? How many moles of carbonate? What is molar mass of the metal?

I tried: n = (PV)/(RT)
n= [(0.123atm)(0.285L)]/[R(298.15 k] = 0.001 moles

My question is: is the pressure given in the aforementioned problem total or CO2 partial, and how can I derive which metal reacted from that? Density?
I tried molar mass, but I got g/L ; I can't convert L to m unless I'm at STP then it's 22.4L/1 mole.
Where am I going wrong?

I believe the decomposition equation would be : xCO3 -> xO + CO2 ?  

[Borek]

Take a look at reaction equation - can you calculate number of moles of carbonate from the number of moles of carbon dioxide?

[vmelkon]

If the problem doesn't mention partial pressure, then 93.6mmHg is the pressure of the CO2.

So you say 0.001 mols of CO2? I don't know, are you sure about n= [(0.123atm)(0.285L)]/[R(298.15 k]?

So, compute the mass of CO2
0.001 mol * (12.01115 + 15.9994 + 15.9994 g/mol) = 0.04400995 g of CO2

You lossed 0.04400995 g
0.211 g - 0.04400995 g = 0.16699005 g of the metal oxide

mass of 0.001 mol of BeO = (9.0122 + 15.9994 g/mol) * 0.001 mol = 0.0250116 g (so we know it is not BeO)
..continue calculations for the other elements of group 2

[opti384]

@ vmelkon: I'd rather find the molar mass of the metal oxide.

[AWK]

Check this result
n= [(0.123atm)(0.285L)]/[R(298.15 k] = 0.001 moles (do not round numbers at this phase of calculations)
and follow Borek advice

[vmelkon]

@ vmelkon: I'd rather find the molar mass of the metal oxide.

That's easy. You know now you have 0.16699005 g of the metal oxide
and
that it is 0.001 mol (same as the CO2 amount).

0.16699005 g/0.001 mol = 166.99005 g/mol for the metal oxide
approx to 3 places = 167 g/mol

You can go a step further from that to find the molar mass of the metal.
166.99005 g/mol - 15.9994 g/mol (for O) = 150.99065 g/mol
approx to 3 places = 151 g/mol

151 g/mol is not close to any element.

[Borek]

0.00144, not 0.001 moles.

That means 87 g/mol.