O level chem questions

StudentQns SG

Hi, i have a mole calculation question here:

when i crystallize MgSO4+H2O to form MgSO4.7H2O, is the mole ratio 1:1?

If so, then if i have 5mole of the crystal, when i use mass/Mr, will the Mr be 24+32+16×4+7(1+1+16). Meaning i multiply the Mr of H2O by 7.
.

UltimaOnline SG

1) I was taught that u could use the 'cross method' for covalent compound to get their formula. so this is wat i dont understand: C4- and O2- cross so its C2O4. then after simplifying its CO2. so is C2O4 the molecular or empirical formula?

What tutor says: C2O4 is the molecular while CO2 is the empirical formula. im not sure if that is true because when we find the molar mass of the compound we will use the molecular formula's to get the molar mass. but we know that the molar mass of carbon dioxide is the addition of all the Ar of CO2 giving us 44.

2)So if CO2 is the molecular formula is it that whenever I do the ‘cross method’ for covalent compounds I need to simplify it to the minimum like for ionic compounds? Then what about stuff like P4O10. If I use the ‘cross method’ I would not get it.

3) Are bases like MgO salts and why if not? I think that they are salts because they contain a metal and a non-metal but I know that salts are supposed to be neutrally charged. So im not sure if this is a salt or not.

4) Are alkalis like NaOH salts and why if not? think they are because they contain a positive metal and a polyatomic ion which is negative in charge. But then again, it is not neutrally charged so im not sure if this is a salt or not.

5) Are acids like HCl salts and why if not? H is positive while Cl is negative so it seems like it is a salt but it is not neutral so again the same from like 3) and 4) so are they considered salts ot not and why?

6) are carbonates salts and why if not? Pretty much the same reason as the others.

7) Precipitation method. Will this method work for any 2 solutions of salts? Like if I put any 2 salt solutions together (when I switch the places of the cat ions and anions a insoluble salt will from) will it always form a solid? Or only some will form the insoluble salt? And how to tell if they will form the insoluble salts if it comes out in a test?

8) is aqueous ammonium NH3(aq) or NH4OH(aq)? My text book says its NH3 but another tutor of mine (a school teacher) says it should be NH4OH. Lastly, when it reacts with acids will it form salt and water like other alkalis do or only a salt?

Thanks so much for helping me. BTW pls answer like 1)Answer –line- 2)Answer –line- and so on. Thanks again for taking the time to help me with this

From prior experience, it's probably better for me to leave 'O' level chemistry questions for others here to help out. So I'll strictly be replying to 'A' level chemistry questions only. Just sayin.

syncopation_music SG

6) are carbonates salts and why if not? Pretty much the same reason as the others.

Thanks so much for helping me. BTW pls answer like 1)Answer –line- 2)Answer –line- and so on. Thanks again for taking the time to help me with this

hi studentqns ,let me try to help you one by one [if i have the time ] i will try my best

(1) empirical formulae is the formulae u obtained thru experimential experience aka going thru experiment the general molecular formulae for oxide of carbon is carbon dioxide [CO2]

C2O4 is the empirical formula

you got quite abit mixed up with the molar mass and empirical formulae .molar mass is for the calculation of SI unit of chemical compound aka mole .recall wHat is it equivalent to ? mass of c-12 .... ?

(2) yes it is

recalled this relationship :

[molecular formulae]= n [empirical formulae]

(8) follow your txtbook .you could have misunderstood what ur tutor say

syncopation_music SG

elaborate on (8) - when ammonia reacted with acid ,ammonia salt will be produced and hYdrogen gas given off.read the qualitative analysis carefully and produced a abbreviated of it in the exam .as there is NO formulae sheet [test for cation and anion ]

my student follow my advice for his combined chem for 2011 nov .he say its effective as all his qa test answer all correct.

Junhwee95 SG

Although this post is already too late as a form of 'help' for the Chemistry Paper 2 by now, nevertheless, I would try to answer these questions as an 'O' level student myself.

My answer to 3) , 4) , 5), 6) : All these are not really salts. If you recall the chapter on 'Salts' , a salt is formed when a metal ion or an ammonium ion replaces the hydrogen ion of an acid. Therefore, salts would normally contain a metal ion such as a sodium ion and an anion which comes from acids , which would commonly be nitrates, chlorides or sulfates .

7) Since the ionic precipitation is one of the methods used to prepare a salt, these are the two conditions for it :

- The salt formed must be an insoluble salt

-The reagents must both be aqueous solutions.

Based on the solubility of salts , you would have to first deduce whether the salt is to be prepared would be soluble or insoluble . ( i.e All chloride salts are soluble except for salts containing lead(II) and silver ions ) .In the method of ionic precipitation, you would come up of two solutions such that one would contain the cation of the insoluble salt which you are preparing, and the other containing the anion. For example, if you are to prepare calcium sulfate which is an insoluble salt, you would use calcium hydroxide and maybe sulfuric acid as the reagents.

Hope these helps !

StudentQns SG

oh so means that all oxides or hyroxides or acids arent salts. so if i see it that way are all have a neutral pH?

Jack007sparow SG

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Jack007sparow SG

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Junhwee95 SG

Yes. Erm , which substances are you referring to ?

SBS n SMRT SG

CO2 is the only form you need to know, so is actually so call empirical formulae, C2O4 is only how u use to deduce CO2

Not needed in O levels to know P4O6 and P4O10, just use criss cross

Nope, salt is form when acid react with a base, O levels all salts are neutral

same as (3)

6) are carbonates salts and why if not? Pretty much the same reason as the others.

Can be

Provided that they are more stable in each other, refer to stability table in electrolysis (displacement), Usually cations are K, Na, Ca, Mg, Al, (C), Zn,Fe,Sn,Pb,(H),Cu, Hg,Ag,Au,Pt, anions are sulfate, nitrate, carbonate, hydroxide (more stable --> least stable

If like say K carbonate and Ba sulfate, they will form K sulfate (soluble) and Fe carbonate (insoluble)

MUG THIS

Both can, NH3 or NH4+OH-, what acid, if HCL, salt NH4Cl formed

Thanks so much for helping me. BTW pls answer like 1)Answer –line- 2)Answer –line- and so on. Thanks again for taking the time to help me with this

See quoted

SBS n SMRT SG

From prior experience, it's probably better for me to leave 'O' level chemistry questions for others here to help out. So I'll strictly be replying to 'A' level chemistry questions only. Just sayin.

can lah, but just know when to stop like dun nid to explain P4O10 and P4O6 (and go on to TM, 3d, 4s orbital energetically accessible, Hindi theory and etc)

StudentQns SG

Hi, i have a mole calculation question here:

when i crystallize MgSO4+H2O to form MgSO4.7H2O, is the mole ratio 1:1?

If so, then if i have 5mole of the crystal, when i use mass/Mr, will the Mr be 24+32+16×4+7(1+1+16). Meaning i multiply the Mr of H2O by 7.

chemtuition SG

syncopation_music SG

Hi, i have a mole calculation question here:

when i crystallize MgSO4+H2O to form MgSO4.7H2O, is the mole ratio 1:1?

If so, then if i have 5mole of the crystal, when i use mass/Mr, will the Mr be 24+32+16×4+7(1+1+16). Meaning i multiply the Mr of H2O by 7.

hi studentqns ,you are wonderful by posting chem question ,but but but ,please help us by NOT EDITING the original question you post in the SUPER PAST else its very confusing to any person reading out there . i dont think your very first post is this water of crystallisations .cheers

StudentQns SG

ok but then i just post my new questions at the bottom? or can i get the mods to delete the other ppl’s post?

syncopation_music SG

ok but then i just post my new questions at the bottom? or can i get the mods to delete the other ppl’s post?

you can post ur new question at the bottom .i dont suggest you remove other ppl's post as we all here to learn .

regarding your question .i hope you can rephrase based on the actual question you wanted to ask .it is abit difficult to answer your question

according to chemical analysis ,yes water of crystallisation may have a proportion of up to 50% which is not uncommon

In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat. ^[1] It is the total weight of water retained by certain salts at a given temperature^[2] and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of a metal complex but which is not directly bonded to the metal ion.

Upon crystallization from water or moist solvents, many compounds incorporate water molecules in their crystalline frameworks.

Compared to inorganic salts, proteins crystallize with unusually large amounts of water in the crystal lattice. A water content of 50 % is not uncommon. The extended hydration shell is what allows the protein crystallographer to argue that the conformation in the crystal is not too far from the native conformation in solution.

http://en.wikipedia.org/wiki/Water_of_crystallization

StudentQns SG

Hi i have some new chem question that I hope to affirm thanks

1) do all ionic compounds have a pH of 7. They are not yet dissolved in water. I think it should be 7 as it does not release H+ or OH- ions at that state. This include salts and oxides.
2) all basic oxides, and acidic oxides have a pH of 7 right? Same reasoning as in 1). N only if basic oxides get a pH of >7 is when they are added to water to form a alkali right?( only reacts if its group 1 oxide and calcium oxide)

Thanks for the help

UltimaOnline SG

Hi i have some new chem question that I hope to affirm thanks

Thanks for the help

Since I specailize at teaching 'A' levels, I won't directly address StudentQns, but I'll comment from a higher 'A' level perspective, to guide those who would guide 'O' level students.

1) pH (ie. -log10[H+] where [H+] refers to molarity of H+ in a solution) applies to solutions only. Therefore to say a solid has such and such a pH is utter nonsense.

2) same reasoning as 1.

When bases (eg. metal ionic oxide) is added to water, hydrolysis (ie. chemical reaction with water) occurs in which the base, in an attempt to stabilize it's negative charge, struggles to grab protons from water; ie. a lone pair on the O2- ion becomes a dative bond pair with a H+ from water. Water is thus deprotonated to generate it's conjugate base the OH- ion. A solution with a greater molarity of OH- over H+ is considered alkaline, and has pH > 7 at rtp.

When acidic species (eg. non-metal covalent oxide) is added to water, hydrolysis (ie. chemical reaction with water) occurs in which the water nucleophile attacks the electrophilic atom of the covalent oxide (since the heteroatom is less electronegative than oxygen), generating species which are considered acidic (since the conjugate base is stabilized by resonance and/or induction), and thus able to release H+ into water (to generate the hydroxonium ion H3O+). Such a solution with a greater molarity of H+ (or H3O+, these two species may be considered equivalent for acid-base calculations at 'A' levels) over OH- is considered acidic, and has pH < 7 at rtp.

StudentQns SG

This is what my tuition teacher said: OH- ions are not the only determining factor which determine the alkalinity of solutions. O2- is considered as a very strong base ion too.
Na2O is alkaline in nature already. Its pH >7. Its a basic oxide.

However, I agree with u, ultimaonline that solids or gases do not have a pH at all since pH is only conceded with solutions and solids and gases are not a solution. And also O2- can’t be an indicator since its pH which compare OH- and H+ right?

Then Na2O can’t have a pH of less than 7, but it doesn’t have a pH at all right?

So I’m confused about what is wrong n right. Personally I think that ultimaonline is right. Thank you

StudentQns SG

Then Na2O can’t have a pH of less than 7, but it doesn’t have a pH at all right?

So I’m confused about what is wrong n right. Personally I think that ultimaonline is right. Thank you