Question
Question: In a gravimetric determination of \[{{{P}}_{{1}}}\] an aqueous solution of dihydrogen phosphate ion ...
In a gravimetric determination of P1 an aqueous solution of dihydrogen phosphate ion H2PO4− is treated with the mixture of ammonium and magnesium ions to precipitate magnesium ammonium phosphate, Mg(NH4)PO4.6H2O. This is heated and decomposed to magnesium pyrophosphate, Mg2P2O7 which is weighted. A solution of H2PO4− yielded 1.054 g of Mg2P2O7. What weight of NaH2PO4 what present originally?
A.1.14 g
B.1.62 g
C.2.34 g
D.1.33 g
Solution
The method used to determine the concentration or mass of a substance by measuring the change in mass is Gravitational analysis. The amount of an analyte can be determined from the mass of the compound and the change in the concentration or mass. NaH2PO4 is also known as monosodium phosphate which is used to detect magnesium ions from salts, used as a food additive.
Complete step by step answer:
The given reactions are:
H2PO4−→Mg(NH4)PO4.6H2OheatMg2P2O7
Mg2P2O7→NaH2PO4
The first reaction shows the formation of magnesium pyrophosphate on heating, whereas in the second reaction sodium dihydrogen phosphate is obtained from magnesium pyrophosphate.
Here the weight of Mg2P2O7 (Magnesium pyrophosphate) =1.054 g
Applying, POAC (Principle of Atom Conservation) on Mg2P2O7 and NaH2PO4 on P atom (since P atom is conserved).
1 mole of NaH2PO4 contains 1 mole of P and 1 mole of Mg2P2O7 contains 2 moles of P.
⇒1×(mol.wtofNaH2PO4)(wt.ofNaH2PO4)=2×(mol.wtofMg2P2O7)(wt.ofMg2P2O7)
⇒120(wt.ofNaH2PO4)=2×2221.054
Here, 120 represents the molecular weight of sodium dihydrogen phosphate and 222 represents the molecular weight of magnesium pyrophosphate.
(Now we will put the values given like the weight of Mg2P2O7 is 1.054 and also we will calculate the molecular masses of NaH2PO4 and Mg2P2O7) .
⇒wt.ofNaH2PO4=2222×1.054×120
Weight of NaH2PO4 is 1.139 approx1.14
(After calculation we get the value for the weight of NaH2PO4 is 1.139 g which is approximately 1.14 g)
Hence, the correct option is (A).
Note:
The POAC (Principle of Atom Conservation) states that the total number of atoms of the reactants should be equal to the total number of atoms of the products.
The POAC comes from the Law of Conservation of Mass.
Mass of atoms of an element in reactant = Mass of atoms of an element in the product.
The number of atoms of element of the reactants = Number of atoms of element of the product.
Moles of atoms of element of the reactants = Moles of atoms of element of the product.