Gareth56

From a book:- Mass differs from electric charge in that it is always positive, and that the forces between masses are therefore always attractive. Electric charge can be both positive and negative, and charges of the same sign repel each other, while charges of opposite sign attract each other. So if mass is always positive why do two masses attract each other and not repel as like repels like?

I thought it was stretching the imagination somewhat too far. I wonder where they came up with this process is known as Ionisation by Magnetic Induction or IMI!!

Could this be true? Wouldn't the magnetic field strength have to be far higher if it would work at all?? Gas Magnet

The world's largest oil tanker comes in at about 565000kg so if there were two of these leviathans 100m appart at sea (calm, no strong currents ) would there be a noticable attraction between them? I calculated a force of about 0.002N.

Thanks. Presumably the above applies to most or all the conc. acids like H2SO4 and HNO3 and bases like NH3? Also is there a difference between vol/vol & wt/vol solutions ? I ask because do you have to take the SG into account if your making up a solution in vol/vol or only if you're using the wt/vol method?

If you want to make up for example a 1Molar HCl solution do you have to take into account the specific gravity of the conc. HCl that you're using? From memory I used to measure out 36.5mls of conc HCL and make that up to 1000ml in a volumetric flask. Is there a formula that you can use to tell you the amount of substance you need to make up a given molarity of solution? It's straight forward if you're using solids because the formula if I recall is:- No. of Moles required = weight in grams/molecular weight e.g. a 0.5M solution of NaOH would be 20grams of NaOH made up to a 1000mls. I think:-) Thanks G56

So how do you work out the amount of electricity you use? Do you need the specific heat capacity of water or something like that?

So it's [marginally] cheaper to use the 2kW kettle. Presumably the difference would be greater if one were using a lower wattage kettle such as those little ones one takes on holiday.

Could someone settle an "difference of opinion" between myself and a friend? He says that it's cheaper to use a 1500W kettle to boil 1L of water than a 2kW. I think you use the same amount of electricity because the 1500kW kettle is heating the water for longer whilst the 2kW is heating for a shorter time thus canceling out the two effects. How can I show him this is the case if of course I'm correct? Ta

Ah those pesky old logs I was told about all those years ago Many thanks

So ow do you do it. Baring in mind that ln2^10^22 is the natural log of 2 raised to the power of 10 to the 22.

What would give 2.135*10^-21 J/K ?

Which is several ordersof magnitude smaller then 0.096J/K Could the 10^22 come into it somewhere. The whole calculation is S = 1.4 x 10^-23J/K x ln2^10^22 = 0.096J/K

I see. I ask because in my book Chemistry in Context it states the following calculation fo entropy:- S = k lnW where W is the number above and k is Boltzman's constant and the answer given is 0.096J/K

How doI calculate this on a calculator? ln2^10^22 this is the natural log of 2 raised to the power of 10 to the 22. Ta

Presumably this is why a petrol/oxygen mixture requires a spark to ignite it? On it's own at 298K the petrol/oxygen mixture is stable with respect to the products of the combustion.

Thank you very much for such a comprehensive explanation. So basically what you're saying is that all the figures for standard enthalpy changes found in tables tell you that the reactions begin at 298K but the reactants may require an input of energy such as a flame as in the case off the Mg + O2 example to get them to go and the reactions themselves are NOT taking place at 298K. Those 'O' levels were a long time ago

OK, thanks anyway.

Thank you. For completeness of the answer sought I don't suppose you know the method of the above technique? How do you work out that you have an enthalpy change of -602kJ/mol after heating the Mg up to 1000*C then allowing it to cool?

If you could possibly indulge me a tad further I'd be grateful. The values of enthalpies of formation in tables are given under STP conditions (they are in my books anyway) which with regard to temperature is 298K. What I don't understand and forgive me for not understanding you concise explanation is how can they quote a value of -602kJ/mol for the formation of MgO at 298K when the reaction doesn't even go at that temperature.

How do you do that then? Also putting the standard symbol in the Delta H isn't necessarily correct then?

I'm brushing up on my chemistry and looking at Enthalpy and it says in my book 'Hill & Holman' that: Standard Conditions are 1 atmosphere pressure, Temperature of 298K and substances being in their normal physical states. Now H&H states that the definition of the Standard enthalpy change of formation of a substances is the heat evolved or absorbed when one mole of the substance is formed from its elements under standard conditions. So in the reaction: Mg(s) + 1/2O2 ----> MgO(s) DeltaH(standard formation) = -602kJ/mol What I don't quite understand, probably because I've forgotten, is that if you want the above reaction to go then you have to burn the Mg in oxygen to get it to react to form MgO so how can the above reaction be under standard conditions if being under standard conditions involves carrying out the reaction at 298K. Surely burning the Mg is carrying out the reaction at temperature higher than 298K. In other words a lump of Mg placed in contact with O2 at 298K will not react but the above reaction states that the value obtained is under standard conditions of 298K.

So why does the US (and some other countries) have a different voltage from the UK & Europe. Is it easier to produce 120V or cheaper? Or is it just down to safety, sticking your fingers into a 120V socket is less dangerous than sticking them into a 240V one?

Many thanks Wouldn't a thicker cord have a greater resistance per foot as there is more copper to resist the flow of the electrons? I have an extension cord on a reel and on it it says that the maximium load while the reel is fully wound up is 720W and when fully unwound 2400W, why the differnece in wattage just becaause the cable is would up? I've always wondered as to why the USA has a different voltage to the UK & Europe and what would happen if I came over for a holiday and plugged in my 1200W hairdryer as it would be drawing twice the current (10A vs 5A) would I blow a fuse or blow the hairdryer up