exam 1
8. an experiment revealed the empirical formula of a compound to be ch₂. in a separate experiment the molar mass of the compound was determined to be 42.2 g/mol. what is the molecular formula for this compound?
a) ch₂ b) c₂h₄ c) c₃h₆ d) c₄h₈

9. the weighted average of the mass of all isotopes in a naturally occurring sample of an element is known as the:
a) atomic number b) atomic mass c) mass number d) density

10. of the following elements, which is not found as diatomic molecule under normal conditions?
a) bromine b) helium c) oxygen d) iodine

11. why do the names of some salts include the charge of the metal ion (as a roman numeral) while others do not?
a) because salts are covalent compounds which follow the law of multiple proportions
b) because salts always include polyatomic ions that have multiple possible charges
c) because the non-metal can have a range of negative charges
d) because the metal can form cations of more than one charge

12. nickel (ni) is a(n):
a) alkali metal b) alkaline earth metal c) halogen d) transition metal

13. which of the following elements will tend to form 2+ ions?
a) lithium (li) b) strontium (sr) c) oxygen (o) d) bromine (br)

14. which of the following elements will tend to form 1– ions?
a) lithium (li) b) strontium (sr) c) oxygen (o) d) bromine (br)

15. what is the correct formula for the compound formed between aluminum and oxygen?
a) alo₂ b) al₂o₃ c) al₃o₂ d) al₂o

16. given the following chemical name, write the chemical formula for each compound. (12)
a) sodium carbonate dihydrate
b) nitrogen tribromide
c) hydrochloric acid
d) nickel(ii) bromide
e) sulfurous acid
f) ammonium sulfate

Question

exam 1
8. an experiment revealed the empirical formula of a compound to be ch₂. in a separate experiment the molar mass of the compound was determined to be 42.2 g/mol. what is the molecular formula for this compound?
   a) ch₂  b) c₂h₄  c) c₃h₆  d) c₄h₈

9. the weighted average of the mass of all isotopes in a naturally occurring sample of an element is known as the:
   a) atomic number  b) atomic mass  c) mass number  d) density

10. of the following elements, which is not found as diatomic molecule under normal conditions?
    a) bromine  b) helium  c) oxygen  d) iodine

11. why do the names of some salts include the charge of the metal ion (as a roman numeral) while others do not?
    a) because salts are covalent compounds which follow the law of multiple proportions
    b) because salts always include polyatomic ions that have multiple possible charges
    c) because the non-metal can have a range of negative charges
    d) because the metal can form cations of more than one charge

12. nickel (ni) is a(n):
    a) alkali metal  b) alkaline earth metal  c) halogen  d) transition metal

13. which of the following elements will tend to form 2+ ions?
    a) lithium (li)  b) strontium (sr)  c) oxygen (o)  d) bromine (br)

14. which of the following elements will tend to form 1– ions?
    a) lithium (li)  b) strontium (sr)  c) oxygen (o)  d) bromine (br)

15. what is the correct formula for the compound formed between aluminum and oxygen?
    a) alo₂  b) al₂o₃  c) al₃o₂  d) al₂o

16. given the following chemical name, write the chemical formula for each compound. (12)
    a) sodium carbonate dihydrate
    b) nitrogen tribromide
    c) hydrochloric acid
    d) nickel(ii) bromide
    e) sulfurous acid
    f) ammonium sulfate

Accepted Answer

### Question 8
# Explanation:
## Step1: Calculate molar mass of empirical formula
The empirical formula is $ CH_2 $. Molar mass of $ C = 12.01 \, g/mol $, molar mass of $ H = 1.008 \, g/mol $. So molar mass of $ CH_2 $ is $ 12.01 + 2\times1.008 = 14.026 \, g/mol $.
## Step2: Find the ratio $ n $
$ n=\frac{\text{Molar mass of compound}}{\text{Molar mass of empirical formula}}=\frac{42.2}{14.026}\approx3 $.
## Step3: Determine molecular formula
Multiply subscripts in empirical formula by $ n $. So molecular formula is $ C_{3}H_{6} $ (since $ 1\times3 = 3 $ for $ C $ and $ 2\times3 = 6 $ for $ H $).
# Answer: c) $ C_3H_6 $

### Question 9
# Brief Explanations:
Atomic number is number of protons. Atomic mass is weighted average of isotopic masses. Mass number is sum of protons and neutrons. Density is mass/volume. So the weighted average of isotopic masses is atomic mass.
# Answer: b) atomic mass

### Question 10
# Brief Explanations:
Diatomic molecules under normal conditions: $ Br_2 $ (bromine), $ O_2 $ (oxygen), $ I_2 $ (iodine). Helium ($ He $) is monatomic as it is a noble gas.
# Answer: b) helium

### Question 11
# Brief Explanations:
Salts with metals having multiple charges (like transition metals) use Roman numerals to indicate charge. Salts of metals with fixed charges (like group 1, 2) don't. Option d explains this: metal can form cations of more than one charge.
# Answer: d) because the metal can form cations of more than one charge

### Question 12
# Brief Explanations:
Alkali metals: group 1. Alkaline earth: group 2. Halogens: group 17. Nickel is in d - block, so transition metal.
# Answer: d) transition metal

### Question 13
# Brief Explanations:
Lithium (Li) forms $ Li^+ $. Strontium (Sr) is group 2, forms $ Sr^{2+} $. Oxygen (O) forms $ O^{2 - } $. Bromine (Br) forms $ Br^- $. So strontium forms $ 2+ $ ions.
# Answer: b) strontium (Sr)

### Question 14
# Brief Explanations:
Lithium (Li) forms $ Li^+ $. Strontium (Sr) forms $ Sr^{2+} $. Oxygen (O) forms $ O^{2 - } $. Bromine (Br) is halogen, forms $ Br^- $ (1 - ion).
# Answer: d) bromine (Br)

### Question 15
# Explanation:
## Step1: Determine charges
Aluminum ($ Al $) has charge $ + 3 $, oxygen ($ O $) has charge $ - 2 $.
## Step2: Use criss - cross method
Formula is $ Al_2O_3 $ (3 from O to Al, 2 from Al to O: $ Al_{2}O_{3} $).
# Answer: b) $ Al_2O_3 $

### Question 16
#### a) sodium carbonate dihydrate
# Explanation:
## Step1: Identify ions
Sodium ion: $ Na^+ $, carbonate ion: $ CO_3^{2 - } $, dihydrate means 2 $ H_2O $.
## Step2: Balance charges
2 $ Na^+ $ balance 1 $ CO_3^{2 - } $, then add $ 2H_2O $. Formula: $ Na_2CO_3\cdot2H_2O $.
# Answer: $ Na_2CO_3\cdot2H_2O $

#### b) nitrogen tribromide
# Explanation:
## Step1: Identify elements
Nitrogen (N), bromine (Br). Tribromide means 3 Br.
## Step2: Write formula
Nitrogen is $ N $, bromine is $ Br $, so $ NBr_3 $.
# Answer: $ NBr_3 $

#### c) hydrochloric acid
# Explanation:
## Step1: Identify ions
Hydrogen ion: $ H^+ $, chloride ion: $ Cl^- $.
## Step2: Write formula
$ HCl $.
# Answer: $ HCl $

#### d) nickel(II) bromide
# Explanation:
## Step1: Identify charges
Nickel(II) means $ Ni^{2+} $, bromide is $ Br^- $.
## Step2: Balance charges
2 $ Br^- $ balance 1 $ Ni^{2+} $, so formula $ NiBr_2 $.
# Answer: $ NiBr_2 $

#### e) sulfurous acid
# Explanation:
## Step1: Identify ions
Hydrogen ion: $ H^+ $, sulfite ion: $ SO_3^{2 - } $.
## Step2: Balance charges
2 $ H^+ $ balance 1 $ SO_3^{2 - } $, so formula $ H_2SO_3 $.
# Answer: $ H_2SO_3 $

#### f) ammonium sulfate
# Explanation:
## Step1: Identify ions
Ammonium ion: $ NH_4^+ $, sulfate ion: $ SO_4^{2 - } $.
## Step2: Balance charges
2 $ NH_4^+ $ balance 1 $ SO_4^{2 - } $, so formula $ (NH_4)_2SO_4 $.
# Answer: $ (NH_4)_2SO_4 $

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QUESTION IMAGE

Question

Explanation:

Question 8

Step1: Calculate molar mass of empirical formula

Step2: Find the ratio \( n \)

Step3: Determine molecular formula

Answer:

Question 9