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Answer to Question #6689 in Physical Chemistry for darren

Question #6689
The mineral hydromagnesite is a hydrated carbonate of magnesium, with the formula Mgx(CO3)y(OH)z.nH2O and a molar mass of 466.0 g/mol. When 1.000g of a pure hydromagnesite is heated to constant mass, solid MgO was obtained. Steam was given off, together with 0.378 g of carbon dioxide. The MgO obtained was then completely dissolved in 50 cm3 of 1.0 mol/dm3 HCl. The resultant solution was transferred into a volumetric flask and diluted to 250 cm3 with deionized water. A 25.0 cm3 aliquot of this solution required 19.00 cm3 of 0.15 mol/dm3 solution of NaOH for complete neutralization.

Calculate the values of x and y.
Hence, or otherwise, deduce the values of z and n.
Write a balanced equation, including state symbols the thermal decomposition of a pure sample of hydromagnesite.
Expert's answer
Mgx(CO3)y(OH)z.nH2O ----->x MgO + y CO2 + (n + z) H20

n
(Mgx(CO3)y(OH)z.nH2O)=1.000/466.0 =0.00215

n(co2)=0.378 /
44=0.00859

n(mgo)=0.0108


in 1000 cm3 is 0.15 mol
in 19 cm3
will be 0.00285
x 0.00285
HCl+NaOH=NaCl + H2O

x=0.00285
but only in 25 cm3

0.00285 * 250/25=0.0285

so if in 1000 cm3 is 1
mol ,in 50 cm3 will be 0.05 mol
HCl which reacted with MgO will be
0.05-0.0285=0.0215

x 0.0215
MgO + 2 HCl= MgCl2 +
H2O

x=0.0215/2=0.0108

n(MgO):n(CO2)=0.0108:0.00859=1:1.25=3:4

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