Italian IMAT 2018 Q 47

The constituents of Ca3(PO4)2 have the following molar masses expressed in g/mol: Ca=40, P=31, O=16. It can be said that 0,125 moles of this compound:

A) contain 1 mole of oxygen atoms
B) contain 7,525 * 1024 molecules
C) have a mass of 155g
D) contain a mass of Ca higher than that of oxygen
E) dissociating completely, they release 0,25 moles of cations

Hi everyone!

I took way to long to solve this one, especially for A) and D)
i would love to see someone’s worked solution to see where i’m getting mixed up

Answer is A)


let’s analyze each answer options:
1). moles oxygen atoms= 0.125 x no. of oxygen atoms= 0.125 x 8= 1
2). molecules = moles(n) x Avogadro’s No. = 0.125 * 6 x 10^23= 7.525 x 10^22 molecules so, it’s incorrect.
3). It has a molar mass of 310g
4). for this option we just need to calculate the constituent mass of ca and o in the compound which are 45g and 128g respectively making it incorrect.
5). for moles of cation we know that it dissociates into 3 moles of ca+2 and 2 moles of PO4^-3 thus, moles cation = 3 * 0.125= 0.375 i hope this is what they mean did you get the same answer for E.
Hope it helps:)


Thank you Ujjwal that’s very helpful!
i also have the same answer for E)
for D) how did you get the 120 and 128 values?

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Hi! i agree it’s confusing

to me they’re asking for the mass in g not the molar mass
using the formula in A) to find n(O)=1 and n(Ca)=0,375 we have 1 * 128 = 128g oxygen and 0,375 * 120 = 45g Calcium

The way you answered it, just from the difference in molar mass you could tell the difference in mass would be the same?

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Okay i guess you’re right i confused it your way of getting the answer for D is the right and as you found the mass of O is way greater than Ca making this option incorrect…
so basically we have to consider m that’s what they are asking for not Mr…

oh okay that clears things up!!
thank you so much for your help!

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