Calcium sulfate occurs naturally as dihydrate gypsum, CaSO4.2H2O. When heated to 160 deg cel, gypsum loses some of its water to give plaster of Paris, which is used for making fine plaster casts and for setting broken limbs.
During the heating process, 100g of gypsum loses 15.7g of water. Calculate the formula of plaster of Paris.
Well, not sure how to start off solving... so pls assist. (do not provide me with solutions, just a brief guide will do, LOL)
thanks
Find no. of moles of gypsum.
Find no. of moles of water removed.
Find no. of moles of water removed per mole of gypsum.
Find formula of plaster of paris.
Originally posted by UltimaOnline:Find no. of moles of gypsum.
Find no. of moles of water removed.
Find no. of moles of water removed per mole of gypsum.
Find formula of plaster of paris.
Sorry UltimaOnline, but it seems like I still can't grasp the concept behind this theory. May I know if the formula of gypsum is CaSO4.2H2O or CaSO4 (without the water)? Even so, I still have problems on how doing the steps above can help me derive the answer. Thank you.
And to everyone else, I've got 2 questions to ask.
1. 10cm3 of a gaseous hydrocarbon was exploded with an excess of oxygen. There was a contraction of 30cm3. A further contraction of 30cm3 took place when the residual gases were subjected to aq. NaOH. All volumes were measured at r.t.p. Calculate the molecular formula of the hydrocarbon.
Is the first contraction due to O2 being reacted away? Anyway, I know the no. of C atoms is 3, but I can't find the no. of H atoms, got 0 H atoms. LOL. Is the 30cm3 in the 2nd line wrongly stated. The answer given is C3H8.
2. 10cm3 of a hydrocarbon, C4Hy, were allowed to react with an excess of O2 at 150deg cel and 1 atm. There was an expansion of 15cm3. Deduce the value of y.
What is the expansion due to?
Sorry, haven't studied the part on eudiometry but I do have a rough idea what it's about. Just can't solve some of the slightly 'twisted' questions for now. Thanks once again.
I'll reply you regarding the plaster of paris. The other problems, if no one else helps you with them within 3 days, then I'll comment on them. (c'mon current 'O' and 'A' level students, feel free participate in discussions for your own practice and benefit!)
"Dihydrate gypsum" is "CaSO4·2H2O".
Find no. of moles of gypsum.
100/(40.1+32.1+4(16)+2(18)) = 0.5807 mol
Find no. of moles of water removed.
15.7/18 = 0.8722 mol
Find no. of moles of water removed per mole of gypsum.
0.8722/0.5807 = 1.5 mol
Find formula of plaster of paris.
CaSO4·(2H2O-1.5H2O) = CaSO4·0.5H2O.
Fyi, btw :
http://wiki.answers.com/Q/What_is_the_chemical_formula_of_Plaster_of_Paris
Originally posted by Audi:
1. 10cm3 of a gaseous hydrocarbon was exploded with an excess of oxygen. There was a contraction of 30cm3. A further contraction of 30cm3 took place when the residual gases were subjected to aq. NaOH. All volumes were measured at r.t.p. Calculate the molecular formula of the hydrocarbon.Is the first contraction due to O2 being reacted away? Anyway, I know the no. of C atoms is 3, but I can't find the no. of H atoms, got 0 H atoms. LOL. Is the 30cm3 in the 2nd line wrongly stated. The answer given is C3H8.
2. 10cm3 of a hydrocarbon, C4Hy, were allowed to react with an excess of O2 at 150deg cel and 1 atm. There was an expansion of 15cm3. Deduce the value of y.
What is the expansion due to?
Sorry, haven't studied the part on eudiometry but I do have a rough idea what it's about. Just can't solve some of the slightly 'twisted' questions for now. Thanks once again.
These questions are vague and badly iterated (it's common to encounter such vague, badly iterated questions from schools, and occasionally even from Cambridge), because they do not unambiguously state whether the contraction/expansion refers to in comparsion with reactant gases or product/residual gases.
For instance, "You combust 10 cm3 of hydrocarbon with 10 cm3 of O2. When the residual gases are cooled to rtp, a contraction of 5 cm3 was observed."
Does the qn mean 5cm3 contraction compared to product/residual gases? (ie. 5cm3 then refers to only water vapour)
Or
Does the qn mean 5cm3 contraction compared to reactant gases? (ie. 5cm3 then refers to the difference between the total volume of reactant O2 + CxHy gases at rtp and the total volume of product/residual gases CO2 + H2O + unreacted O2, at rtp)
If the question is ambiguous and you interpret it in the former, you get one answer; if you interpret it in the latter, you get another answer.
An exam smart student, will give both answers, BUT WITH QUALIFICATIONS. In other words, explain to the examiner that the quesiton is ambiguous, show clear working for each case / interpretation, and how you arrived at the two alternative answers.
You will score full credit / marks, if you give both answers. If you only give one answer, you have about 50% chance that your interpretation is the one expected by the examiner.
Of course, if you do not qualify/explain your alternative answers, you will get zero marks, because the examiner thinks you're trying to cheat by giving more than one answer.
So be exam smart, and secure your marks.
(Audi, it's admirable that you're studying up on your own ahead of your school topics. But it's not feasible to expect everything to be explanied / taught clearly to you over an internet forum. If you still do not understand how to do these questions, or topics ahead of your school, then either wait for your school to teach you, or/and attend private tuition to keep ahead of your school.)