| Author | Message | ||
     Chio U Kim Username: cuk Registered: 04-2003 |
Dear S4 Chemistry Students, All questions about Chemistry are welcome ! Do work hard and get the best results for the coming exam. | ||
| 991035 Username: 991035 Registered: 04-2003 |
我想問下呢, 我明 weak acid 同 weakly acid 既分別, 但係就唔知用 mei 形式去考。 仲有可唔可以教多次點樣計 1A book o個 d 好煩既 calculation 呀? thx | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
There are three main types of calculation : 1. % by mass of element in compound E.g. % by mass of Ca in CaCO3 = 40/(40+12+3x16)=40% 2. relative atomic mass from mass numbers and % abundance of isotopes E.g. rel. atomic mass of Cl = 35x75% + 37x25% = 35.5 3. Calculation based on balanced equation. E.g. Calculate the mass of CO2 produced when 12 g of carbon completely burn in oxygen. Step 1 : Write a balanced equation for the reaction: C + O2 ® CO2 Step 2 : Calcuate the no. of moles of C reacted. No. of moles of C reacted = mass/molar mass = 12g/12g = 1 mol Step 3 : According to the balanced equation 1 mole of C gives 1 mole of CO2 No. of moles of CO2 produced = 1 mol Mass of CO2 produced = no. of mole x molar mass = 1x(12+2x12)g = 44g | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
There are three important formulae for calculating number of moles. For aqueous solutions, no. of moles = molarity x volume of solution(in dm3) For pure substances (solids, liquids or gases), no. of moles = mass/molar mass = no. of particles/Avogagro's constant | ||
| Real me Username: 991097 Registered: 04-2003 |
For salt, is there only AgCl, BaSO4, CaSo4,PbSO4,PbCl2 are insoluble?? Then they are ppt.??? | ||
| 991145 Username: 991145 Registered: 04-2003 |
我想問.... What mass of silver Ag is formed when 6.8g of magnesium Mg reacts with silver nitrate solution? 我只係知道equation 係: Mg+2AgNO3=>2Ag+Mg(NO3)2 但係我就唔識計la........ | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
Refer to Chem 2000 book 1B p.128. All halides (chlorides/bromides/iodides) are soluble except silver halides(AgCl/AgBr/AgI) and lead(II) halide (PbCl2/PbBr2/PbI2). All sulphates are} soluble except those of lead, barium and calcium (PbSO4/BaSO4/CaSO4) Also, All carbonates are insoluble except Na2CO3, K2CO3 and (NH4)2CO3 All hydrogencarbonates are soluble | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
No. of moles of Mg reacted = mass/molar mass = 6.8g/24.3g = 0.28 mol According to the balanced equation : Mg + 2AgNO3 ® 2Ag + Mg(NO3)2 1 mole of Mg produced 2 moles of Ag. Therefore, No. of moles of Ag formed = 2x0.28 mol = 0.56 mol Mass of Ag formed = no. of moles x molar mass = 0.56x107.9g =60.42g | ||
| 991035 Username: 991035 Registered: 04-2003 |
But no need to care about the salt solution????? if the question is asking you to find the nass of the solution after reacting. then do we need to care about the metal calculation? | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
It does not make sense to calculate the mass of a solution. You will only be asked to calculate the concentration or the molarity of a soltuion. Can you calculate the molarity of the Mg(NO3)2(aq) formed when 6.8 g of Mg react completely with 25 cm3 of silver nitrate solution? (You can assume that the volume of the solution remains unchanged throughout the reaction) | ||
| 991035 Username: 991035 Registered: 04-2003 |
is it 50 cm3? | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
You were asked to calculate the molarity, not volume! | ||
| 991035 Username: 991035 Registered: 04-2003 |
.............. 咁個 50cm3 o岩唔o岩呀?..... 唔好意思 | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
Wrong! 50 cm3 is a volume, not concentration! | ||
| 991085 Username: 991085 Registered: 04-2003 |
咁係咪all sulphites 都係insoluble | ||
| 991035 Username: 991035 Registered: 04-2003 |
請你開估吧............ | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
Mg(s) + 2AgNO3(aq) ® 2Ag(s) + Mg(NO3)2(aq) 1 mole of Mg produced 1 mole of Mg(NO3)2. Therefore, No. of moles of Mg(NO3)2 formed = 0.28 mol Since the volume of the solution remains unchanged throughout the reaction, volume of Mg(NO3)2 solution = 25 cm3 = 0.025 dm3 So, the molarity of Mg(NO3)2(aq) formed = no. of mole/volume in dm3 = 0.28/0.025 = 11.2 M | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
Amendment!! All sulphates are soluble in water except lead(II) sulphate, barium sulphate and calcium sulphate. You don't have to know the solubility of most sulphites. However, you should know if sodium sulphite or potassium sulphite or ammonium sulphite is soluble in water. Do you know why? | ||
| 991035 Username: 991035 Registered: 04-2003 |
咁可唔可以咁計呀?? Mg(s) + 2AgNO3(aq) →2Ag(s) + Mg(NO3)2(aq) mol of Mg = 0.28mol Molarity of Mg reacte with 2AgNO3 is 0.28molX1000 /25cm3 =11.2 M assume the volumn doesn't change. The molarity of Mg(NO3)2 is also 11.2 M | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
Mg is insoluble in water. It does not make sense to calculate the concentration of Mg. | ||
| Real me Username: 991097 Registered: 04-2003 |
copper and H2SO4 have reaction , is it because of the redox le?? | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
Copper is oxidized to copper(II) ions by concentrated sulphuric acid and concentrated sulphuric acid is reduced to sulphur dioxide by copper. Remember : copper shows no reaction with dilute sulphuric acid. | ||
| 小布丁 Username: 991097 Registered: 04-2003 |
How to make the standard sloution?? I don't understand the example in the book ar!! | ||
| 眼淚 Username: 991160 Registered: 04-2003 |
chio sir 你上面所講的 3. Calculation based on balanced equation. E.g. Calculate the mass of CO2 produced when 12 g of carbon completely burn in oxygen. Step 1 : Write a balanced equation for the reaction: C + O2 ® CO2 Step 2 : Calcuate the no. of moles of C reacted. No. of moles of C reacted = mass/molar mass = 12g/12g = 1 mol Step 3 : According to the balanced equation 1 mole of C gives 1 mole of CO2 No. of moles of CO2 produced = 1 mol Mass of CO2 produced = no. of mole x molar mass = 1x(12+2x12)g = 44g 有冇其他既例子? 書邊度有? | ||
| 眼淚 Username: 991160 Registered: 04-2003 |
係咪書既112,113? | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
1. Calculation based on balanced equation ® Chem 2000 book 1A p.168-169 2. Calculation involving the formulae No. of moles = mass/molar mass = no. of particles/Avogadro's constant ® Chem 2000 book 1A 109-113 | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
991097 A standard solution (a solution of known concentration) can be prepared by either 1. dissolving an accurately known amount of solid in distilled (or deionized) water and making up the solution to an accurately known volume (E.g 250 cm3) in a volumetric flask. (see activity 12.7 B) OR 2. diluting a standard solution.(see activity 12.7C) For example, a 0.1 M HCl solution can be prepared by diluting 25.0 cm3 of 1.0 M HCl solution to 250 cm3. | ||
| 小布丁 Username: 991097 Registered: 04-2003 |
I want to ask some reaction (obsurve): 1. 2M hydrochloric acid + solid copper(II) carbonate 2. 2M nitric acid + copper foil 3. 2M sodium chloride solution use Bunsen burner to boil 4. 2M sodium hydroxide solution + 2M copper(II) chloride solution 5. distilled water use Bunsen burner to boil | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
1. the solid dissolves to give colourless gas bubbles and a blue solution. 2. copper foil dissolves to give a blue solution and a brown gas. 3. a white solid is left behind. 4. a blue precipitate is formed. 5. Nothing left | ||
| 小布丁 Username: 991097 Registered: 04-2003 |
For 1,is the colourless gas is CO2?? For 2, wt is the brown gas?? For 3, the white solid is salt?? | ||
| Chio U Kim Username: cuk Registered: 04-2003 |
The colourless gas is CO2 The brown gas is NO2 The white solid is NaCl |