Questions....Help

[Rosie]

Urgent help with gravimetric questions needed

Is molecule the same as atoms. If a question aks for one molecule of a compound, would you use the n=N/Na relationship.
such as: One molecule of butane (C4H10) would have a mass, in grams, of
A. 58/6*1023
B. 58*6*1023
C. 58
D. 6*1023/58

A 2L sample of a gaseous hydrocarbon is burnt in excess oxygen. The only products of the reaction are 8L of CO2(g) and 10L of H2O(g) all at 100 degrees and 1 atm pressure. The formula of the hydrocarbon is:

An important ore of uranium is pitchblende. One of the uranium coumpounds present in this ore has the formula U3O8. What mass of this compund would contain 0.1mol of oxide oins? (Does that mean there is 0.1 mol for 1 oxide atom)

The mass of magnesium chloride, MgCl2, that would contain 0.25mol of choloride ion is:

0.148g of a metal carbonate exacts reacts with 20ml of 0.1M HCl. The formula of the metal carbonate is:

Hydrogen and chlorine react according to the equation
H2 + Cl2 -----> 2HCl
3 mole of H2 and 2 mole of Cl2 are placed in a vessel and sealed. When reaction is complete the vessel will contain:
A. 5 mole of HCl
B. 6 mole of HCl and 1 mole of Cl2
C. 4 mole of HCl and 1 mole of Cl2
D. 4 mole of HCl and 1 mole of H2

[beezy4eva]

Is molecule the same as atoms. If a question aks for one molecule of a compound, would you use the n=N/Na relationship.
such as: One molecule of butane (C4H10) would have a mass, in grams, of
A. 58/6*1023
B. 58*6*1023
C. 58
D. 6*1023/58

molecules and atoms aren't the same thing =P But yeh you can use the same formula
First find the Mr of C4H10. This gives you the mass of one mole of the substance. To find out how much one molecule would weigh, divide this by avogadros number.

A 2L sample of a gaseous hydrocarbon is burnt in excess oxygen. The only products of the reaction are 8L of CO2(g) and 10L of H2O(g) all at 100 degrees and 1 atm pressure. The formula of the hydrocarbon is:

Since they're all at the same temperature and pressure, we can use the volumes as an indication of the formula (and can figure out the oxygen gas required also):
2hydrocarbon+ 13O2 --> 8CO2 + 10H2O
from this we can see that 2 molecules of hydrocarbon would contain 8C and 20H, therefore one would have 4C and 10H, so the molecule must be butane, C4H10.

[Rosie]

An important ore of uranium is pitchblende. One of the uranium coumpounds present in this ore has the formula U3O8. What mass of this compund would contain 0.1mol of oxide oins? (Does that mean there is 0.1 mol for 1 oxide atom)

The mass of magnesium chloride, MgCl2, that would contain 0.25mol of choloride ion is:

0.148g of a metal carbonate exacts reacts with 20ml of 0.1M HCl. The formula of the metal carbonate is:

Hydrogen and chlorine react according to the equation
H2 + Cl2 -----> 2HCl
3 mole of H2 and 2 mole of Cl2 are placed in a vessel and sealed. When reaction is complete the vessel will contain:
A. 5 mole of HCl
B. 6 mole of HCl and 1 mole of Cl2
C. 4 mole of HCl and 1 mole of Cl2
D. 4 mole of HCl and 1 mole of H2

[Mao]

An important ore of uranium is pitchblende. One of the uranium coumpounds present in this ore has the formula U3O8. What mass of this compund would contain 0.1mol of oxide oins? (Does that mean there is 0.1 mol for 1 oxide atom)

then multiply by Mr.

The mass of magnesium chloride, MgCl2, that would contain 0.25mol of choloride ion is:

0.148g of a metal carbonate exacts reacts with 20ml of 0.1M HCl. The formula of the metal carbonate is:

the reaction of carbonate with acid:

hence

*assuming that the metal carbonate is in the form of or , hence*



taking away Mr of carbonate from that, we end up with 88, which is close to Strontium.

Hydrogen and chlorine react according to the equation
H2 + Cl2 -----> 2HCl
3 mole of H2 and 2 mole of Cl2 are placed in a vessel and sealed. When reaction is complete the vessel will contain:

the stoichiometric ratio of reactants here is 1:1, hence Cl2 is the limiting reactant
for every mole of Cl2 used, 2 moles of HCl is produced, therefore a total of 4 mol HCl is produced.
the remaining unreacted reactant, 1 mol of H2, remains.

[Rosie]

Q. Equal masses of the two gases oxygen (O₂) and sulfur dioxide (SO₂) are placed in separate vessels. Both vessels have the same volume and are at the same temperature. The pressure exerted by the oxygen is 100kpa. The pressure, in kpa, exerted by the SO₂ is:


Q. The molar volume of oxygen, in L, at 1atm and 100 degrees is:


Q. The label on a 5L container of pool acid states that it contains 33% w/w of HCl. Calculate the mass of the acid present in the container, in kg, if the density of the acid is 1.17g ml^-1


Q. 20ml of a 0.10M solution of HCl reacts with 20ml of 0.3M KOH solution. The concentration of potassium ions in the resultant solution, in mol per litre is:

When undergoing a titration or volumetric analysis, the conical flask can only be washed with deionised water and no other solution. But wouldn't this dilute the base. Shouldn't the conical flask be dried properly then.

[Mao]

drying properly works, but in normal laboratory conditions, that is very hard to achieve [water vapour in the atmosphere], and takes a lot of effort [you'd have to put the flasks in the oven every time its used and washed]

washing with water, however, makes very little difference [if any], and is practical, hence preferred

[Mao]

it shouldn't make any difference at all, unless its not deionised water and theres some impurities in there that react with the titrant

no it shouldnt, as the acid/base are already aqueous solutions

on another note and to avoid starting a new thread, how exactly does the flame of AAS atomise all the metal ions? where would they pick up the extra electrons?

not part of the course. we only need to know that it reduces the ion to ground state, not how

[Rosie]

back to my questions.....please

[unknown id]

Q. Equal masses of the two gases oxygen (O₂) and sulfur dioxide (SO₂) are placed in separate vessels. Both vessels have the same volume and are at the same temperature. The pressure exerted by the oxygen is 100kpa. The pressure, in kpa, exerted by the SO₂ is:

the molar mass of SO₂ is approximately double that of O₂ and given that there are equal masses of each present, the number of moles of SO₂ will be half that of O₂. As pressure exerted is directly proportional to the number of moles present, SO₂ will exert exactly half the pressure than that of O₂, which is 50 kPa.

Q. The molar volume of oxygen, in L, at 1atm and 100 degrees is:

[Mao]

Q. The label on a 5L container of pool acid states that it contains 33% w/w of HCl. Calculate the mass of the acid present in the container, in kg, if the density of the acid is 1.17g ml^-1

since the density is



hence

Q. 20ml of a 0.10M solution of HCl reacts with 20ml of 0.3M KOH solution. The concentration of potassium ions in the resultant solution, in mol per litre is:

K⁺ is a spectator ion. The volume is doubled -> concentration halved.

:. 0.15M

Q. The molar volume of oxygen, in L, at 1atm and 100 degrees is:

a method that avoids this formula:

[Rosie]

Can I ask for people's overall equation for the oxidation half equation of
CH₃CH₂OH + H₂O -----> CH₃COOH + 4H⁺ + 4e^-
and reduction half equation of
Cr₂O₇^2- + 14H⁺ + 6e^- ----> 2Cr³⁺ + 7H₂O

Q. Can alkenes also undergo combustion reactions too

Do we also have to study cracking because it has it in a few revision books that I have

[orangez]

No, you don't need to learn about cracking - it has been shifted to unit 4. And yes, alkenes undergo combustion.

[beezy4eva]

Can I ask for people's overall equation for the oxidation half equation of
CH₃CH₂OH + H₂O -----> CH₃COOH + 4H⁺ + 4e^-
and reduction half equation of
Cr₂O₇^2- + 14H⁺ + 6e^- ----> 2Cr³⁺ + 7H₂O

(CH₃CH₂OH + H₂O -----> CH₃COOH + 4H⁺ + 4e^-) x 3
(Cr₂O₇^2- + 14H⁺ + 6e^- ----> 2Cr³⁺ + 7H₂O) x 2
overall equation:
3CH₃CH₂OH + 3H₂O + 2Cr₂O₇^2- +  28H⁺ + 12e^- ------> 3CH₃COOH + 12H⁺ +  12e^- + 4Cr³⁺  + 14H₂O
balancing everything it becomes:
3CH₃CH₂OH +  2Cr₂O₇^2- + 16H⁺ -----> 3CH₃COOH + 4Cr³⁺ + 11H₂O

[Mao]

No, you don't need to learn about cracking - it has been shifted to unit 4. And yes, alkenes undergo combustion.

you should know, however, what it is, what the products can be [ethene + other stuff], and how to balance equations given the reactant and products.

e.g. decane is cracked to form ethene and one other hydrocarbon. write a chemical reaction formula for this.

[orangez]

Sure, but I don't think you're gonna be examined on it on Thursday.