Chemistry - 9202 OxfordAQA

Use Of Amount Of Substance In Relation To Masses Of Pure Substances

Bayani Gaba-gaba

A white crystalline solid arrives in a laboratory with nothing written on the jar. Nobody knows what it is. Within an hour an analyst can hand back its formula, and the only instrument that really mattered was a balance. That is a strange claim on the face of it: a balance reports masses, and a formula is a statement about how many atoms of each kind are locked together. Getting from one to the other is the single most useful piece of arithmetic in the whole of chemistry, and it is what this lesson is about.

You will start by learning to add up a formula, turning a string of symbols and subscripts into one number that stands for the mass of the whole unit. From there you will work out what fraction of a compound is the element you actually care about, which is how a farmer chooses between two fertiliser sacks and how a mining company decides whether an ore is worth digging. Then comes the reverse trip, and it is the best part: given nothing but masses measured on a bench, you will reconstruct the formula of a substance you have never met, then sharpen that formula until it names one compound and one compound only.

Manufura

  1. The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula. Students are expected to use relative atomic masses in the calculations specified in the subject content. Students should be able to calculate the relative formula mass (Mr) of a compound from its formula.
  2. The percentage by mass of an element in a compound can be calculated from the relative atomic mass of the element in the formula and the relative formula mass of the compound.
  3. The empirical formula of a compound can be calculated from the masses or percentages of the elements in a compound. Students should be able to calculate empirical formulae and molecular formulae.

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Takardar Darasi

Suppose you are handed a sample of a compound built from just two elements, and you are told its mass and how much of that mass is each element. Nothing else. No spectrometer, no reference book, no helpful colleague. It sounds like far too little information to name the substance, and for most of the history of chemistry people agreed with you. The breakthrough came from realising that atoms of different elements have fixed, comparable masses, so a mass on a balance is a disguised count of atoms. Undo the disguise and the formula falls out.

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Nazarin Darasi

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Za ka gamu da haɗe-haɗen nau'ikan tambayoyi, ciki har da tambayoyin zaɓi da yawa, tambayoyin gajeren amsa, da tambayoyin rubutu. Kowace tambaya an ƙirƙira ta da kyau don auna fannoni daban-daban na iliminka da ƙwarewar tunani mai zurfi.

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  1. What is the relative formula mass of calcium carbonate, CaCO3? Relative atomic masses: C = 12, O = 16, Ca = 40. A. 68 B. 88 C. 100 D. 116 Answer: C
  2. What is the percentage by mass of oxygen in water, H2O? Relative atomic masses: H = 1, O = 16. A. 11.1 B. 16.0 C. 50.0 D. 88.9 Answer: D
  3. A compound contains 1.4 g of nitrogen and 0.3 g of hydrogen. What is its empirical formula? Relative atomic masses: H = 1, N = 14. A. NH B. NH2 C. NH3 D. N3H Answer: C
  4. Which of these is the empirical formula of glucose, C6H12O6? A. CHO B. CH2O C. C2H4O2 D. C3H6O3 Answer: B
  5. A compound has the empirical formula CH2 and a relative formula mass of 70. What is its molecular formula? Relative atomic masses: H = 1, C = 12. A. C2H4 B. C3H6 C. C4H8 D. C5H10 Answer: D

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Littattafan da ake ba da shawarar karantawa

OxfordAQA International GCSE Chemistry (9202)
OxfordAQA International GCSE Chemistry (9202)
Sunaƙa: Student Book
Mai wallafa: Oxford University Press
Shekara: 2016
ISBN: 978-0-19-837589-0
Bayanan bayanin.: The endorsed student book for the 9202 specification, covering the full subject content with required practicals and exam guidance.
OxfordAQA International GCSE Combined Sciences Chemistry (9204)
OxfordAQA International GCSE Combined Sciences Chemistry (9204)
Sunaƙa: Student Book
Mai wallafa: Oxford University Press
Shekara: 2017
ISBN: 978-0-19-842308-9
Bayanan bayanin.: The chemistry half of the Combined Science series. A lighter parallel treatment of the same content, useful for a second explanation.

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