Homework 21
- Page ID
- 28878
Q1.64
Question:
Isotopes are atoms that have the same number of protons but differ in the number of neutrons. Write the chemical symbols for each of the following isotopes:
a) the chlorine isotope with 18 neutrons
b) the chlorine isotope with 20 neutrons
c) the calcium isotope with 20 neutrons
d) the neon isotope with 11 neutrons
Strategy:
Step 1: Determine the given element's atomic number from the periodic table.
Step 2: Add the number of neutrons given in the question to the atomic number to calculate the mass number of the isotope.
Step 3: The mass number will be represented in the upper left corner of the chemical symbol, while the atomic number will be represented in the lower left corner.
Helpful Hints:
- A chemical symbol is written as \[_{Z}^{A}\textrm{X}\] . The A stands for the number of protons and neutrons, also known as the mass number. The Z stands for the number of protons, also known as the atomic number.
- Subtracting Z from A will give you the number of neutrons.
- Example: \[_{19}^{39}\textrm{K}\] --> 39-19= 20 neutrons
- Therefore, adding the given number of neutrons to the atomic number, Z, gives you the mass number.
Solution:
a) \[_{17}^{35}\textrm{Cl}\]
b) \[_{17}^{37}\textrm{Cl}\]
c) \[_{20}^{40}\textrm{Ca}\]
d) \[_{10}^{21}\textrm{Ne}\]
Q5.80
Question:
For each substance, convert the given molecules to mass in grams:
a) 3.2 x 1024 Cl2 molecules
b) 8.25 x 1018 CH2O molecules
c) 1 carbon dioxide molecule
Strategy:
Step 1: Convert the given molecules to moles by dividing by Avogadro's number.
Step 2: Multiply the number of moles by the Molar Mass of the substance to determine the grams.
Step 3: Through correct Dimensional Analysis, the molecules and moles will cancel to leave grams.
\[\frac{molecules}{Avogadro's}\rightarrow moles\rightarrow moles \cdot MM\rightarrow grams\]
Helpful Hints:
- Avogadro's number: 6.022 x 1023 mol of a substance
- Molar mass (MM): the weight of a substance in the units g/mol, found by adding the atomic masses of each element.
- Example: MM of H2O --> H(1.008 x 2) + O(15.999)= 18 g/mol
Solution:
a) 3.8 x 1051 grams
b) 4.11 x 1045 grams
c) 7.31 x 1025 grams