Chemistry - Mole concept

bonkysleuth SG

there's this kind of question where they give you a hydrated salt, heat the salt to remove the water of crystallisation from it. I think the equation goes something like ____(salt's name)._H2O

How do you solve such questions? I did these questions before and looked through the required steps before the test today but I met with a hiccup midway throught the question. My answer was different from that of my friends'. could someone explain how you find out the "n" value of the H2O.

n is the unknown number, i think it's the mole ratio.

UltimaOnline SG

A mixture of MgSO₄.7H₂O and CuSO₄.5H₂O is heated until a mixture of the anhydrous salts, is obtained. If 5.0g of the hydrated mixture when heated gives 3.0g of the anhydrous salts, calculate the % by mass of CuSO₄.5H₂O in the initial hydrated mixture.

Ans : 73.9%

UltimaOnline SG

1) Weigh the anhydrous and hydrated salts. (the masses of both will be provided by the question.)

2) Calculate the molar mass of the anhydrous salts (from its formula).

3) Find the no. of moles of anhydrous salts. (no. of moles = sample mass / molar mass.)

4) Using algebra, equate mass of hyrated salts (by stoichiometry) with mass of hydrated salts (observed from experiment).

bonkysleuth SG

1) Weigh the anhydrous and hydrated salts. (the masses of both will be provided by the question.)

2) Calculate the molar mass of the anhydrous salts (from its formula).

3) Find the no. of moles of anhydrous salts. (no. of moles = sample mass / molar mass.)

4) Using algebra, equate mass of hyrated salts (by stoichiometry) with mass of hydrated salts (observed from experiment).

Come to think of it, I think my question goes something like this.

They give you a particular mass for the hydrated salt (eg 50.0g) and next they say water of crystallisation is removed by heating the salt to give the remaining anhydrous salt. They then tell you mass of anhydrous salt, say,28.0g.

Formula goes something like FeCl3.nH2O

question asks to find value of n. what i did was, i find the mass of the water that has been removed. in the case i created, the water that has evaporated is 22.0g. then i used 22 /18, where 18 is the Mr of H2O. i think i missed out some steps in between. do you know what they are?

UltimaOnline SG

Bonky, did you even read my post above? The detailed steps required are indicated therein.

UltimaOnline SG

Bonky, in future, please at least make a visible effort to understand what I posted previously in response to your question (ie. if you're unsure what I meant by a particular statement, ask "Correct me if I'm wrong, but is this what you meant when you said...?"), or it will seem as though you're wasting my time in replying you.

In case you failed to understand the 4 steps described above; this is what you need to do - after you've calculated the no. of moles of water evaporated off, divide it by the no. of moles of anhydrous salts, that will give you the value of 'n'. (I expect you to know why this works... if you don't immediately know why, think over it first, then ask here to confirm your ideas).