To find:
To calculate:
a. The formula masses of CdS and CdSe
b. Atoms of Se, if a nanoparticle of CdSe contains 2.7 x 107 atoms of Cd
c. Gram of Cd and S in 4.3 x 10-15 g of CdS
Answer to Problem 2.112QA
Solution:
a) Formula mass of CdS is 144.475 g/mol and formula mass of CdSe is 191.37 g/mol
b) Atoms of Se in CdS = 2.7 x 107
c) Mass of Cd in the CdS sample is 3.3 x 10-15 g and mass of S in the nanoparticle = 9.5 10 -16 g.
Explanation of Solution
Formula and concept: The problem can be solved using a series of unit conversions.
A mole is the SI unit of amount chemical substance. When writing units, it is written as “mol”.
Molar mass of a compound is the sum of the
Molar mass of the substance is treated as a conversion factor to interconvert grams and moles. Mole ratio within the compound is treated as ratio of atoms in that compound.
For example: 1 mole of CdS contains 1 mole of Cd and 1 mole of S
Similarly, 1 molecule of CdS contains 1 atom of Cd and 1 atom of S
A mole is the SI unit of amount chemical substance. When writing units, it is written as “mol”.
Given information:
We are given
a. Molecular formulae for CdS and CdSe
b. Atoms of CdS are given as 2.7 x 107 in the sample
c. Mass of CdS is given as 4.3 x 10-15 g
Calculations:
a. Atomic mass of Cadmium = 112.411 g per mol
Atomic mass of S = 32.065 g per mol
Molecular formula = CdS, the molar mass for CdS will be:
Thus, the molar mass for CdS is 144.476 g per mol
Similarly, the molar mass for CdSe is calculated as:
Thus, the molar mass for CdSe is 191.371 g per mol
b. 1 molecule of CdSe = 1 atom of Cd and 1 atom of Se
This can be treated as a conversion factor to find atoms of Se from atoms of Cd as follows:
2.7
c. Molar mass of CdS is 144.475 g/mol that is 1 mole of CdS sample weighs 144.475 gram.
We know that 1 molecule of CdS contains 1 atom of Cd
Atomic weight for Cd is 112.41 g/mol that is 1 mole of Cd weighs 112.41 gram.
So, the conversion factors needed are:
Hence, the set up would be:
Thus, the mass of Cd atoms in the given sample of CdS is 3.3 x 10-15 g.
On similar lines, we can calculate mass of S atoms as follows:
Thus, mass of S atoms in the given sample of CdS is 9.5 x 10-16 g.
Conclusion:
Molar mass of a substance is the sum of atomic weights of each element present in that substance. Just like mole-to-mole ratio, you can write the molecule-to-atoms ratio for a given compound. Molar mass of the compound is used to convert between moles and mass and vice a versa.
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