Norini Jaafar Chemistry: July 2011

Wednesday 27 July 2011

To make a hypothesis

How to construct or make a hypothesis?

First, you must understand that a hypothesis is a statement that relates a manipulated variable with a responding variable.
Therefore, you must be able to identify the manipulated variable and the responding variable for an experiment.

For example, in experiment to determine the effect of temperature on the rate of reaction between sodium thiosulphate and sulphuric acid.

The manipulated variable is temperature, and the responding variable is rate of reaction.

Therefore, the statement of hypothesis is: When the temperature of sodium thiosulphate increases, the rate of reaction with sulphuric acid increases.

Monday 25 July 2011

Operational Definition

Operational definition is a statement that shows the "what you do and what you observe".

For example, the operational definition for coagulation of latex:

The "what you do" is... when acid is added to latex..

The "what you observe " is.... white solid is formed.

Therefore the operational definition for the coagulation of latex is ..When acid is added to latex, white solid is formed. (SPM 2008)

The operational definition for the neutralisation of acid or alkali could be... When acid is added to alkali with a few drops of phenolphthalein, the pink solution changes to colourless.

The operational definition for the reactivity of Group 1 elements towards water:
When a metal which is lower down in Group 1is put into a basin filled with water, the movement of the metal on the water surface is faster // more vigorous// the flame produced is bigger or brighter.

Teacher Norini hopes that you girls and boys have some idea now the meaning of operational definition. Well, you can always ask Teacher Norini if you don't.

Friday 22 July 2011

Constructs in Paper 3

In Paper 3 Chemistry 4541/3, there are several constructs to be understood and solved.
For example, making hypothesis, aim @ problem statement, naming variables, to state the operational definition and so on.

Question for the day!

What is operational definition?

For example: Give the operational definition for the neutralisation of acid and alkali.

Teacher Norini will give the answer later....

Monday 18 July 2011

Difference in pH value between hydrochloric acid and ethanoic acid.

Answer
1. Hydrochloric acid is a strong acid.
2. Therefore, acid can ionise completely in water
3. to produce high concentration of hydrogen ion
4. Ethanoic acid is a weak acid.
5. This acid can ionise partially in water
6. to produce low concentration of hydrogen ion
7. When the concentration of hydrogen ion is higher, the pH value is lower

Salam boys and girls!

QUESTION FOR THE DAY.

Hydrochloric acid 1.0 mol dm-3 has 2 pH value.
Ethanoic acid 1.0 mol dm-3 has 6 pH value.

Explain the difference in pH value for the acids

Sunday 17 July 2011

Salam. Sorry for the late reply for the answers to previous questions.
Ok, the answer to why alloy is harder than its's pure metal is:
1. Atoms of foreign element added to metal are of different size from atoms in pure metal.
2. These atoms will disrupt the orderly arrangement of the atoms in metal.
3. When force is applied, the foreign atoms will reduce the sliding of the atoms over each other.
4. Thus making alloy harder and stronger.

Pure metal is soft, malleable and ductile because:
1. Atoms are of the same size
2. Atoms are arranged in layers and orderly manner.
3. When force is applied the layers of atoms can easily slide over each other

Note: make sure you know how to draw the diagram for the arrangement of atoms in alloy and pure metal

Wednesday 13 July 2011

QUESTION FOR THE DAY

WHY IS ALLOY HARDER THAN IT'S PURE METAL?

Teacher Norini will give the answer tomorrow.

Tuesday 12 July 2011

More homework on Chemical Formulae and Equations

1(a) What is the meaning of empirical formula?

.....................................................................................................................................................

[1 mark]

(ii) A carbon compound X has an empirical formula of CH₂ and a molar mass of 70 g mol^-1. Determine the molecular formula of compound X.

[3 marks]

(b) A group of students carried out an experiment to determine the empirical formula of magnesium oxide.

(i) Draw the apparatus set up for the experiment.

[2 marks]

Diagram 1 shows the results of the experiment.

Mass of crucible + lid = 24.88 g

Mass of crucible + lid + magnesium ribbon = 25.12 g

Mass of crucible + lid + magnesium oxide = 25.28 g

Diagram 1

(ii) Based on Diagram 1, complete Table 1 and determine the empirical formula of magnesium oxide.

Element	Mg	O
Mass (g)
Number of moles
Simplest ratio of moles
Empirical Formula

Table 1

[6 marks]

(iii) Write the chemical equation for the reaction in the experiment.

......................................................................................................................................................

[1 mark]

......................................................................................................................................................

[1 mark]

(d) Why this experiment is not suitable to be used to determine the empirical formula of copper(II) oxide?

......................................................................................................................................................

[1 mark]

2. A group of students carried out an experiment to determine the empirical formula

of magnesium oxide.

Result :

Mass of crucible + lid = 24.0 g

Mass of crucible + lid + magnesium ribbon = 26.4 g

Mass of crucible + lid + magnesium oxide = 28.0 g

(a) What is meant by empirical formula?

…………………………………………………………………………………………………............................

........................…………………………………………………………………………………………………. [1 mark]

(b) Based on the above results,

Calculate the empirical formula of magnesium oxide.

[3 markah]

................................................................................................................................................................................

[1 mark]

(d) Why was the crucible lid opened once in a while during the experiment?

__________________________________________________________________________________

[ 1 mark]

(e) Metal X is placed below hydrogen in the reactivity series. You are required to carry out an experiment to determine the empirical formula of the oxide of metal X. The apparatus provided are combustion tube, glass tube, cork, Bunsen burner and porcelain dish.

(i) draw a labelled diagram of the set-up of the apparatus for the experiment.

[2 marks]

(ii) Describe the steps that should be taken to ensure that all the air in the combustion tube has

been expelled.

_______________________________________________________________________________

[3 marks]

Homework for the weekend

TOPIK: CHEMICAL FORMULAE AND

EQUATION

1. The equation below represents the reaction to extract aluminium from aluminium oxide.

2Al₃O₃ → 4Al + 3O₂

What is the mass of aluminium that can be extracted from 102 g of aluminium oxide?

[Relative atomic mass: O, 16 ; Al, 27]

13.5 g C. 54.0 g
27.0 g D. 108.0 g

4. XCO₃ → XO + CO₂

The equation above shows the action of heat on the carbonate salt of metal X.

How many moles of XCO₃ are needed to produce 4.0 g of oxide X?

[Relative atomic mass: C, 12; O, 16; X, 64]

0.03 C. 0.08
0.05 D. 0.09

5.. What is the number of molecules in 1 mole of ammonia, NH₃?

[Avogadro’s constant: 6.0 x 10²³ mol^-1]

A. 1.5 x 10²³ molecules

B. 6.0 x 10²³ molecules

C. 1.2 x 10²⁴ molecules

D. 2.4 x 10²⁴ molecules

6. A compound with the formula X₂CO₃ has a relative formula mass of 138.

What is the relative atomic mass of X?

[Relative atomic mass: C=12, O=16]

39 C. 78
69 D. 110

7. The table shows the mass of elements M and O in an oxide, and the relative atomic mass of elements M and O.

Element	M	O
Mass/g	5.6	2.4
Relative atomic mass	56	16

The empirical formula for oxide M is

M₂O₃ C. MO₂
M₂O D. MO

8. 5 g of element X reacted with 8 of element Y to form a compound with the formula XY₂

What is the relative atomic mass of element X?

[Relative atomic mass of Y=80]

A. 25 C. 50

B. 40 D. 100

9. Which of the following statement is true for one mole of a substance?

A. 1 mol of copper contains 6.02 x 10²³ molecules

B. 1 mol of oxygen gas contains 6.02 x 10²³ atoms

C. 1 mol of water contains the same number of atoms as in 12g of carbon-12

D. 1 mol of carbon dioxide contains the same number of molecules as the number of atoms in 12g of carbon-12

10. Table below shows two elements and their respective relative atomic mass.

The letters used are not the actual symbol of the elements.

Element	Relative atomic mass
X	24
M	12

Which of the following is true about the atoms of elements X and M?

A. The mass of 1 mol of X is twice the mass of 1 mol of M

B. 1 mol of X has the same mass as 1 mol of M

C. The mass of an atom of X is 24 g and the mass of an atom of M is 12 g

D. The number of mole in 12 g of atom X is equal to the number of mole in 12 g of atom M.

11. A balloon contains 6.02 x 10²³ of gas particles. What is the number of moles

of the gas in the balloon?

A 0.5 mol C 3.0 mol

B 1.0 mol D 6.0 mol

12. One mole of a substance is defined as the quantity of a substance that contains the same number of particles in m g of element Y. What are m and Y?

	m	Y
A	2	Hydrogen - 1
B	12	Carbon - 12
C	14	Nitrogen - 14
D	16	Oxygen - 16

13. When 6.4 g oxide of M reacted completely with hydrogen gas, 4.48 g of metal M is produced. What is the empirical formula of the oxide?

[Relative atomic mass:O,16; M,56]

A MO C M₂O

B MO₂ D M₂O₃

14. Which of the following gases contains 0.4 mol of atoms at room temperature and pressure?

[1 mol of gas occupies the volume of 24 dm³at room temperature and pressure]

A 4.8 dm³ He C 4.8 dm³ SO₃

B 4.8 dm³ H₂ D 4.8 dm³ CO₂

15. The following equation shows the decomposition reaction of copper(II) carbonate when heated at room temperature and pressure.

CuCO₃ → CuO + CO₂

Which of the following is not true when 1 mol of copper(II) carbonate is decomposed?

[Relative atomic mass: C = 12, O=16, Cu = 64 and 1 mol of gas occupies the volume of 24 dm³ at room temperature and pressure.]

A 1 mol of copper(II) oxide is formed

B 1 molecule of carbon dioxide gas is

given off

C 80 g of copper(II) oxide is formed

D 24 dm³ of carbon dioxide gas is given off

16. Which of the following contains 6.02 x 10²³ atoms?

A 1 mol of nitrogen gas

B 1 mol of chlorine gas

C 1 mol of ammonia gas

D 1 mol of neon gas

17. What is the number of moles of copper(II) nitrates in 56.4 g of copper(II) nitrate, Cu(NO₃)₂?

[Relative atomic mass: O=16, Cu=64, N=14]

A 0.30 mol C 0.45 mol

B 0.32 mol D 3.33 mol

18. Sodium reacts with chlorine to form sodium chloride.

2Na + Cl₂ → 2NaCl

19. What is the mass of sodium chloride formed when 2.30 g of sodium reacts with excess chlorine?

[Relative atomic mass: Na=23, Cl=35.5]

A 2.93g C 9.40g

B 5.85g D 11.70g

20. The relative molecular mass of M₂(SO₄)₃ is 342. What is the relative atomic mass of element M?

[Relative atomic mass: O=16, S=32]

A 27 C 118

B 53 D 123

21. A sample of iron oxide contains 5.6 g of iron and 2.4 g of oxygen. What is the empirical formula of this compound?

[Relative atomic mass: O=16, Fe=56]

A Fe₂O₃ C FeO₂

B Fe₃O₄ D FeO

22. What is the minimum mass of zinc required to react with excess hydrochloric acid to produce 240 cm³ of hydrogen gas at room conditions?

[Molar volume of gas = 24 dm³ mol^-1 at room conditions;

Relative atomic mass: Zn = 65]

A 0.33 g C 1.30 g

B 0.65 g D 6.50 g

END OF QUESTIONS

THANK YOU

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