Phys Sc 430

Pretest 2.2 Solutions

1. Avogadro was one of the first people to use the word "molecule".

What is the difference between a mole and a molecule?

A molecule is two or more atoms chemically bonded to one another.

A mole consists of 6.02 X 10²³ molecules or particles of anything.

Mole comes from a Latin word meaning lump. Molecule means little lump.

2. Find the molar mass of ….

a. Ca(OH)₂

40 + 2(16) + 2(1) = 74 g/mole

b. beryllium nitrate

Be(NO₃)₂

9 + 2(14) + 6(16) = 133 g/mole

3. Convert the following:

a. 300 g of Mg = __________moles

300 g (mole/24 g) = 12.5 moles

b. 1 g of water = __________moles

1 g(mole/18 g) = 0.056 moles

c. 2 moles of O₂ = ________grams

2 moles (32 g/mole) = 64 g

d. 2 moles of O₂ = ________molecules

2 moles (6.02 X 10²³ molecules/mole) = 1.2 X 10²⁴ molecules

4. Monty Mole found a mole of gold(Au). If gold sells for

$12 per gram, what is the value of Monty’s mole of gold?

1 mole Au = 197 g

197 g($12/g) = $2364

5. Two molecules is how many moles?

2 molecule (mole/6.02 X 10²³) = 3.33 X 10^-24 moles

6. Given: 4 NH_3(g) + 5 O_2(g) -----> 4 NO_(g) + 6 H₂O_(g)

A. How many moles of water will be produced if 5 moles of ammonia, NH_3(g), react?

or 5 moles NH₃ (5 O₂/ 4 NH_3(g)) = 7.5 moles O₂

x = 7.5 moles

B. How many grams of oxygen reacted if only one mole of NO reacted?

x = 1.25 moles of oxygen

1.25 moles (32g/mole) = 40 g

C. How many grams of ammonia will react with 2 grams of oxygen gas?

2 g (mole/32 g) = 0.0625 moles

x = 0.05 moles NH₃

0.05 moles NH₃( 17g/mole) = 0.85 g

7. In the reduction of CuO experiment, why was mass seemingly "lost"?

Cu O was turned into Cu, which is an element, so if it’s weighed by itself, it’ll weigh less than CuO. The oxygen is gone.

8. If 20% of an element has a mass of 260 units, and the rest has a mass of 263, what is atomic mass of that imaginary element?

0.20(260) + 0.80(263) = 262.4

9. Flash has a mole of red dye in his left hand and a mole of green dye in his right hand.

Trick question:

a. Which hand holds more molecules? Neither; each has an Avogadro’s number of molecules.

b. Which hand holds more mass if the red stuff is selenium and the green stuff is bismuth? Bi (larger molar mass)

Don’t dye your hair with that stuff!!!

10. Balance:

a. 2 Al_(s) + Fe₂O_3(s) à Al₂O_3(s) + 2 Fe_(l)

b. 4 K + O₂ à 2 K₂O

c. C₁₁H₂₄ + 17 O₂ à 11 CO₂ + 12 H₂O

d. 6 Ca(NO₃)₂ + 2 Na₃PO₄ à 2 Ca₃(PO₄)₂ + 6 NaNO₃

e. CuS + 2 NaOH à Cu(OH)₂ + Na₂S

extra. Complete the table, given the fact that the average atomic mass of Z = 301.5.

Isotope	Mass number	Percent Abundance
^299Z	299	20
^301Z	301	50
^???Z	???	???

0.20*299 + 0.50*301 + 0.30*x = 301.5

x = 304 u

Review Section

11. Which are covalent compounds (two types of atoms bonded together)?

a. BF₃_____

b. H₂O₂____

c. O_{3_________}

d. Ca(OH)_{2_________}

e. air______

12. What is the difference between a compound and an element?

A compound consists of two or more types of elements bonded to another. An element only has one type of atom.

13. Recopy and complete the following table.

Family	Common Charge	3 examples	Reacts with…(give 1 example)
Alkali metals	+1	Li,Na,K	halogens
Alkaline earth metals	+2	Be,Mg,Ca	water
Halogens	-1	F,Cl,Br	Most metals
Noble or Inert gases	0	He,Ne,Ar	Almost nothing

14. Name the following:

a. MgS magnesium sulfide

b. KOH potassium hydroxide

c. O₂ (diatomic)oxygen

d. Ga₂(SO₄)₃ gallium sulfate