CS C1 H Calculating masses

Transcription

1 Name: CS C1 H Calculating masses Date: Time: 27 minutes Total marks available: 27 Total marks achieved:

2 Questions Q1. (a) Complete the sentence by putting a cross ( ) in the box next to your answer. The particles in atoms are electrons, neutrons and protons. The mass of an electron is A greater than the mass of a neutron B the same as the mass of a proton C smaller than the mass of a proton D the same as the mass of a neutron (b) The atomic number of oxygen is 8. The mass number of an atom of oxygen is 17. Describe the number and type of particles in the nucleus of this atom..... (c) Sulfur and oxygen are both in group 6 of the periodic table. Explain, in terms of their electronic configurations, why they are both in group (d) An atom of phosphorus contains 15 electrons. Describe how these 15 electrons are arranged in a phosphorus atom..... (e) Phosphorus oxide is a compound that contains covalent bonds. (i) Describe what is meant by a covalent bond..... (ii) The formula of a molecule of phosphorus oxide is P 4O 10 Give the empirical formula of this oxide.. (1) (1) (Total for Question = 10 marks) Q2. 50 cm 3 of potassium hydroxide solution of concentration 40 g dm 3 is needed for an experiment. Calculate the mass of potassium hydroxide that must be dissolved in water to make 50 cm 3 of solution of this concentration.

3 mass of potassium hydroxide =... g (Total for question = 2 marks) Q3. Calcium nitrate contains calcium ions and nitrate ions. Calculate the relative formula mass of calcium nitrate, Ca(NO 3) 2. (relative atomic masses: Ca = 40, N = 14, O = 16) relative formula mass =... (Total for question = 2 marks) Q g of sulfur dioxide is provided. Calculate the number of sulfur dioxide molecules, SO 2, in this sample. (relative atomic masses: O = 16.0, S = 32.0; Avogadro constant = mol 1 ) (3)

4 number of molecules =... (Total for question = 3 marks) Q5. A compound of sulfur was analysed to determine its empirical formula. (i) State the meaning of the term empirical formula. (1) (ii) A compound of sulfur and fluorine contains 4.8 g of sulfur and 17.1 g of fluorine. Calculate the empirical formula of this compound. You must show your working. (relative atomic masses: F = 19, S = 32) (3) empirical formula =... (Total for question = 4 marks) Q6. A solution is made by dissolving calcium chloride in water g of calcium chloride are dissolved in water. The volume of the solution is made up to 500 cm 3. Calculate the concentration, in mol dm 3, of calcium chloride, CaCl 2, in this solution. (relative atomic masses: Cl = 35.5, Ca = 40.0)

5 (3) concentration =...mol dm 3 Q7. Magnesium reacts with water in the form of steam as shown in the equation. Mg + 2H 2O Mg(OH) 2 + H g of magnesium reacts with sufficient steam for a complete reaction to form 5.8 g of magnesium hydroxide and 0.2 g of hydrogen. Show, by calculation, that the law of conservation of mass applies to this reaction. (relative atomic masses: H = 1.0, O = 16, Mg = 24) (3) (Total for question = 3 marks) Examiner's Report

6 Q1. (b) Many candidates answered this question very well giving the expected answer of 8 protons and 9 neutrons in the nucleus. Candidates sometimes just stated 8 protons, 9 neutrons and 8 electrons, implying, probably without intending to do so as they did not make a clear enough distinction between the nucleus and the rest of the atom, that the nucleus contained all three sub-atomic particles. Poor maths was evident in the processing of the numbers of particles: candidates also confused atomic number and mass number and therefore obtained incorrect values for the numbers of protons and of neutrons. Unfortunately in this question, many candidates had not comprehended the significance of the information about the oxygen atom given in the question. This question was about the oxygen-17 isotope and was deliberately chosen so it was clear how the numbers of protons and neutrons had been obtained. Some candidates thought they should point out what they perceived as an error and gave the answer of 8 protons and 8 neutrons they achieved one mark for this. Doing this they showed that they did not understand the difference between the terms 'relative atomic mass' (as given on the periodic table) and 'mass number' (as referred to in the question), since only the mass number could give the total number of protons and neutrons. Evidence of poor maths skills. One mark scored for the correct number of protons. Several answers were seen like this. It was not clear if they understood where the electrons were located in an atom. Answers like this scored one mark, as the presence of the electrons stopped the awarding of the second mark.

7 Here is an example of an answer where the candidate had wrongly corrected the mass number of the oxygen atom. In this case the answer then went on to describe the electron arrangement and showed they did not understand the arrangement of particles in an atom. This answer scored 0 marks. Although here the candidate pointed out a perceived error, they then chose to ignore it and obtained two marks for the correct answer. Results Plus: Examiner Tip Learn the difference between relative atomic mass and mass number. Learn the basic properties of the sub-atomic particles - mass, relative charge and location. (c) This question was answered generally well, with many scoring 2 marks. Of the correct answer the majority were of the type stating that both oxygen and sulfur were in group 6 because their atoms had 6 electrons in the outer shell and the minority were of the type where their atoms both needed two more electrons to fill the outer shell. Common errors included references to them being in group 6 as they were both gases or noble/nobel [sic] gases. There was frequent reference to the atomic number and mass number e.g. those of sulfur being double that of oxygen: 8 16 or Often a reference was made to oxygen and sulfur having same properties, qualities or reactivity or to them both being non-metals, gases or non-conductors. In these instances, the candidates were not answering the question as given.

8 This did not answer the question and scored 0 marks. This was a very good answer scoring 2 marks. One of a number of common errors seen in the marking of this question. 0 marks given. Results Plus: Examiner Tip Understand how the electronic configuration of an atom of an element is related to the position of that element in the periodic table. (d) Generally, this question was answered well; most candidates scored both marks sometimes by the inclusion of a correct electronic configuration diagram. Occasionally seen was references to shells of atoms e.g. 2 atoms on first shell. There were some vague responses such as the electrons were 'around the nucleus' or 'in circles dotted around the nucleus'. Some candidates gave incorrect shells such as 2, 10, 5 with incorrect numbers of electrons that the shells could hold. The answer was helped with the inclusion of the electronic configuration diagram. This

9 was given 2 marks. Although there was a clear electronic configuration diagram to aid this answer, there was some confusion as to where the electrons were located. Unfortunately the candidate had thought the inner circle of the diagram was part of the nucleus, and so only the mark for the correct number of electrons in the outer shell could be given. Results Plus: Examiner Tip Learn the maximum number of electrons that can be held in an electron shell and how the electronic configuration can be worked out. (e) (i) Of the candidates who scored at least one mark for this question, the greater majority obtained 1 mark for the idea of sharing electrons between two atoms. Only a few indicated that a pair of / two electrons were shared. Many others had indicated a sharing of electrons, but between elements rather than between atoms and so could not be given that mark. A number of candidates had confused ionic with covalent bonding and wrote about electrons being transferred. There is confusion here between ionic and covalent bonding. This answer was given 0 marks.

10 One mark was given for the sharing of electrons. No mention was made of the number of electrons in the covalent bond. Results Plus: Examiner Tip Know the difference between covalent and ionic bonding and how the two types of bonding are formed. (e) (ii) Only a small number of candidates knew that the empirical formula meant the simplest ratio of the atoms in the formula of a substance. Many calculated the formula mass or contrived some other form of response, but most left this question unanswered. Q2. No Examiner's Report available for this question Q3. No Examiner's Report available for this question Q4. No Examiner's Report available for this question Q5. No Examiner's Report available for this question Q6. Some very well laid out answers were seen, as in the example shown, and some not so clear answers that were correct. Unfortunately, the working for many with an incorrect answer was unclear making it difficult or impossible for examiners to award part marks. Common errors included the incorrect formula mass 75.5 (CaCl) or (Ca2Cl), and dividing by 500 (cm 3 ) rather than 0.5 (dm 3 ).

11 The correct answer, 0.2, is on the answer line. This example is ideally set out. Q7. No Examiner's Report available for this question Mark Scheme Q1. (a) (b) Answer Acceptable answers Mark C smaller than the mass of a proton (1) An description linking ignore references to electrons in shells / 8 protons (1) charges on particles (and) 17-8/9 if electrons in neutrons (1) nucleus max 1

12 (c) Explanation linking (both have) same number (of electrons) in outer shell(1) 6 (electrons in outer shell) (consequent on first point) (1) protons and neutrons with incorrect numbers (1) correct electronic configurations or diagrams alone max 1 'they both have 6 in the outer shell' scores both marks allow 'both need 2 (more) (electrons) to fill outer shell' for both marks (d) A description to include suitable diagram in place of 2.8(1).5(1) 2.8 (in 1 st and 2 nd electrons in shells / shell)(1) orbits / rings(1) 5 (in outer shell)(1) (e)(i) A description to include can be shown in a diagram of a electron(s) covalent bond any shared (1) mention of ions pair(s) of / two scores zero (electrons) (1) (e)(ii) P 2O 5 Reject P2O5 / P 2 O 5 (1) Q2. Q3. Q4.

13 Q5. Q6.

14 Q7.