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(a)
Interpretation:
The formula and molecular mass of sodium dichromate are to be determined.
Concept introduction:
The molecular formula of a compound is a representation which suggests the total number of atoms of each element the compound is made of.
The molecular mass of a compound is defined as the sum of the atomic masses of all the atoms present in it.
The general formula to calculate the molecular mass of a compound is,
Molecular mass of the compound=[Σ(number of atoms of the element)(atomic mass of the element)] (1)
(a)
Answer to Problem 2.103P
The molecular formula for sodium dichromate is Na2Cr2O7. Its molecular mass is 261.98 amu.
Explanation of Solution
Dichromate is a polyatomic ion. The formula for the dichromate ion is Cr2O72−. The dichromate ion carries a charge of −2. Sodium belongs to the 1A(1) group in the periodic table and hence loses one electron to form Na+(sodium ion). To balance the −2 charge on the dichromate ion, the ion combines with two sodium ions carrying +1 charge each.
Hence the molecular formula for sodium dichromate becomes Na2Cr2O7.
The formula to calculate the molecular mass of Na2Cr2O7 (sodium dichromate) is,
Molecular mass=[(2)(atomic mass of sodium)+(2)(atomic mass of chromium)+(7)(atomic mass of oxygen)] (2)
Substitute 22.99 amu for the atomic mass of sodium, 52 amu for the atomic mass of chromium and 16 amu for the atomic mass of oxygen in equation (2).
Molecular mass=[(2)(22.99 amu)+(2)(52 amu)+(7)(16 amu)]=45.98 amu+104 amu+112 amu=261.98 amu
The molecular formula for sodium dichromate is Na2Cr2O7. Its molecular mass is 261.98 amu.
(b)
Interpretation:
The formula and molecular mass of ammonium perchlorate are to be determined
Concept introduction:
The molecular formula of a compound is a representation which suggests the total number of atoms of each element the compound is made of.
The molecular mass of a compound is defined as the sum of the atomic masses of all the atoms present in it.
The general formula to calculate the molecular mass of a compound is,
Molecular mass of the compound=[Σ(number of atoms of the element)(atomic mass of the element)] (1)
(b)
Answer to Problem 2.103P
The molecular formula for ammonium perchlorate is NH4ClO4. Its molecular mass is 117.49 amu.
Explanation of Solution
Ammonium and perchlorate are polyatomic ions. The formula for the perchlorate ion is ClO4−. Ammonium ion is represented by the formula NH4+. Ammonium and perchlorate ions carry the charges +1 and −1 respectively. Hence they combine in the 1:1 ratio to form a molecule of ammonium perchlorate.
Hence the molecular formula for ammonium perchlorate phosphate becomes NH4ClO4.
The formula to calculate the molecular mass of NH4ClO4 (ammonium perchlorate) is,
Molecular mass=[(1)(atomic mass of nitrogen)+(4)(atomic mass of hydrogen)+(1)(atomic mass of chlorine)+(4)(atomic mass of oxygen)] (3)
Substitute 14.01 amu for the atomic mass of nitrogen, 1.008 amu for the atomic mass of hydrogen, 35.45 amu for the atomic mass of chlorine and 16 amu for the atomic mass of oxygen in equation (3).
Molecular mass=[(1)(14.01 amu)+(4)(1.008 amu)+(1)(35.45 amu)+(4)(16 amu)]=14.01 amu+4.032 amu+35.45 amu+64 amu=117.49 amu
The molecular formula for ammonium perchlorate is NH4ClO4. Its molecular mass is 117.49 amu.
(c)
Interpretation:
The formula and molecular mass of magnesium nitrite trihydrate are to be determined
Concept introduction:
The molecular formula of a compound is a representation which suggests the total number of atoms of each element the compound is made of.
The molecular mass of a compound is defined as the sum of the atomic masses of all the atoms present in it.
The general formula to calculate the molecular mass of a compound is,
Molecular mass of the compound=[Σ(number of atoms of the element)(atomic mass of the element)] (1)
(c)
Answer to Problem 2.103P
The molecular formula for magnesium nitrite trihydrate is Mg(NO2)2⋅3H2O. Its molecular mass is 170.38 amu.
Explanation of Solution
Nitrite is a polyatomic ion. The formula for the carbonate ion is NO2−. The nitrite ion carries a charge of −1.
Magnesium belongs to the 2A(2) group in the periodic table and hence loses two electrons to form Mg2+(magnesium ion). To balance the +2 charge on the magnesium ion, the ion combines with two nitrite ions carrying −1 charge on each.
While naming compounds containing polyatomic ions, when two or more of the same polyatomic ion are present, the ion appears in parentheses. The total number of the polyatomic ion is written as a subscript outside the parentheses.
In magnesium nitrite, two nitrite polyatomic ions associate with one magnesium ion. Hence the molecular formula becomes Mg(NO2)2.
Magnesium nitrite trihydrate is the hydrated form of the ionic compound magnesium nitrite. Upon hydration, the molecule of magnesium nitrite bonds with three molecules of water to form a trihydrate molecule. In the molecular formula, the water molecules are shown after a centred dot. Hence the formula for magnesium nitrite trihydrate becomes Mg(NO2)2⋅3H2O.
The formula to calculate the molecular mass of Mg(NO2)2⋅3H2O (magnesium nitrite trihydrate) is,
Molecular mass=[(1)(atomic mass of magnesium)+(2)(atomic mass of nitrogen)+(6)(atomic mass of hydrogen)+(7)(atomic mass of oxygen)] (4)
Substitute 24.31 amu for the atomic mass of magnesium, 1.008 amu for the atomic mass of hydrogen, 14.01 amu for the atomic mass of nitrogen and 16 amu for the atomic mass of oxygen in equation (4).
Molecular mass=[(1)(24.31 amu)+(2)(14.01 amu)+(6)(1.008 amu)+(7)(16 amu)]=24.31 amu+28.02 amu+6.048 amu+112 amu=170.38 amu
The molecular formula for magnesium nitrite trihydrate is Mg(NO2)2⋅3H2O. Its molecular mass is 170.38 amu.
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Chapter 2 Solutions
CHEM 212:STUDENT SOLUTION MANUAL
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