[Auroturtle]

Using a 0.50 M calcium chloride solution, determine the mass of sodium carbonate and potassium iodide in a mixture of the two substances.

Can somebody PLEASE help me, I've been working on this for at least 2 hours and have gotten no where.

[Nobby]

Not enough information

But generally Calcium reacts with Carbonate and precipitate to CaCO₃

with the amount of Calciumcarbonate you also know then the amount of sodiumcarbonate, this you have to substract from the mass of the mixture.

[Borek]

Using a 0.50 M calcium chloride solution, determine the mass of sodium carbonate and potassium iodide in a mixture of the two substances.

I guess you are asked to devise a method to determine the composition of a mixture you are given, yes?

Take a sample, weight it. Dissolve. What happens when you add CaCl₂? Filtrate?

[Auroturtle]

Not enough information

But generally Calcium reacts with Carbonate and precipitate to CaCO₃

with the amount of Calciumcarbonate you also know then the amount of sodiumcarbonate, this you have to substract from the mass of the mixture.

How can you tell the amount of sodium carbonate by looking at the calcium carbonate? Aren't they seperate?

[Borek]

Write reaction equation for calcium carbonate precipitation.

[Auroturtle]

CaCl₂ + 2KI   CaI₂ + 2KCl

CaCl₂ + NaCO₃   CaCO₃ + 2NaCl

So I'm thinking, pour a specific amount of CaCl₂ onto a filtrate containing the two substances. Wait a little, and what I'll have is a precipitate (CaCO₃, right?). Now I don't understand how the mass of this could help me find NaCO₃

[sjb]

CaCl₂ + 2KI   CaI₂ + 2KCl

CaCl₂ + NaCO₃   CaCO₃ + 2NaCl

So I'm thinking, pour a specific amount of CaCl₂ onto a filtrate containing the two substances. Wait a little, and what I'll have is a precipitate (CaCO₃, right?). Now I don't understand how the mass of this could help me find NaCO₃

What does the balanced equation tell you about the reaction? How many moles of calcium carbonate do you form from, say, one mole of sodium carbonate?

[Auroturtle]

Okay, say I have 1 mole of calcium chloride, and calcium carbonate is 1.5 mol, does this mean that sodium carbonate is 0.5 mol?

[Borek]

Have you heard about limiting reagents?

You don't need a "specific amount" of CaCl₂, you need "excess amount".

There will be no CaI₂ precipitate, it is a well soluble salt.

Check out what stoichiometric coefficients mean.

[Auroturtle]

So what would the limiting reactact be? I'm thinking Na₂CO₃ because there are less mols? My teacher is a fail, btw.

[Nobby]

No everything is equal

1mol CaCl₂ reacts with 1 mol Na₂CO₃ and forms 1 mol CaCO₃ and 2 mol NaCl

You use an excess of CaCl₂ to get all soda converted to CaCO₃. From the precipitate you get the weight after filtering and drying. With the molecular weight you know how many moles and these are equal to the moles of the soda. You convert back to the mass to the soda. You need as additional information the mass of the mixture. By Substraction you get the mass of potassiumiodide as well.

[Auroturtle]

Okay I think I've got this.

I'm going to need to use like 18.9mL of CaCl₂, so that it reacts with all of the Na₂CO₃, I could use more but then I'd be wasting it. I'll mix it with 1g of the mixture in a beaker. I'll then take this and put it over a filtrate, because according to the solubility rules (and google, as well as you guys), CaCO₃ will form a precipitate. When we leave it to evaporate overnight, only the solid will be left.

Now I'm thinking all I have to do is find the g of CO₃, setting up a proportion with the molar masses. Lets say the total mass was 0.9g, so it's like (0.9/x) = (100.09/60.01). x = 0.5396 will be the grams of CO₃, and then I can simply make a same proportion back with Na₂CO₃ to find Na₂, which will be (x/0.5396)=(105.99/60.01). Now I add up both Na_{2 and} CO₃, and I have one answer, subtract that from 1g and I have the other.

If you read all of that, can you tell me if this is right?

[Nobby]

Not 100% right.

So the mixture is 1g.  You add excess volume of the CaCl₂ and get CaCO₃. I dont know were 18,9 ml comes from?

You determine the weight of the CaCO₃.  In your example 0,9 g

Now you calculate the moles.  0,9g/100 g/mol = 0,009 mol  This is equal to Na₂CO₃.  You calculate the mass 0,009 mol *106 g/mol = 0,954 g

The mixture was 1g . So the mass of KI is then 1 g - 0,954g = 0,046 g

In your calculation you used 60 g/mol and assume to calculate CO₃^2- but your mass what you had filtered is the CaCO₃.

[Auroturtle]

how do you know the number of mols in CaCO₃ = Na₂CO₃?

[Nobby]

becaus the reaction tells you:

Na₂CO₃ + CaCl₂ => CaCO₃ + 2 NaCl

1 Na₂CO₃ is equal to 1 CaCO₃

0,009 Na₂CO₃ is equal to 0,009 CaCO₃