Chem Questions

[charmanderp]

Ammonium nitrate is actually NH4NO3

The amount of nitrogen is 1.4/28=0.1mol. Using this value, we calculate the mass off each of the products in the ammonium nitrate sample.

Nitrogen: 1.4g
Hydrogen: n(N)*2*1=0.2g
Oxygen: n(N)*3/2*16=2.4g

Adding that all together we get 4g. Then if we recall that it's only 80% pure, we realise we've only equated 80% of the mass. So multiply by 5/4 to get D, 5.0g.

[horizon]

Ammonium nitrate is actually NH4NO3

haha, that's right! Thanks.

Q7) When water is added to a solution of NaOH, the pH of the solution will
A. rise
B. fall
C. remain the same.
D. become neutral.

Answer=B, why?

[Shenz0r]

It doesn't rise because you dilute the solution, hence the concentration of OH- ions will decrease instead of increasing. Lower concentration of OH- = lower pH. 

It doesn't remain the same either, because the concentration of OH- changes due to dilution.

And the pH will not become 7 unless there is also a H+ concentration of 10^-7 M.

Thus B is the best answer.

[pi]

High pH = basic. Water is "neutral" and hence dilutes the solution. Hence pH decreases towards neutral (it will never become acidic through this method btw).


edit: beaten

[horizon]

Understand, thanks.

Q8) In a titration, 0.055g of a monoprotic acid needs 30.45 mL of 0.1M NaOH for neutralisation.  The monoprotic acid must be:
A. HCl
B. HNO3
C. CH3COOH
D. CH3CH2COOH

Answer: A.

Q9)
An important use of sulfuric acid is for the manufacture of superphosphate.  The equation for this
reaction is
Ca3(PO4)2(s) + 2H2SO4(aq) + 4H2O(l) → Ca(H2PO4)2(s) + 2CaSO4.2H2O(s)
Here sulfuric acid is acting as
A an acid.
B an oxidant.
C a reductant.
D a dehydrating agent.

Answer: A

Help please!

[charmanderp]

For question 8, take note of the fact that we have a monoprotic acid as well as a monobasic alkali. This means that the amount of acid (in mol) will be equal to the amount of alkali (in mol).

For question two, look at what H2SO4 does? It donates protons.

[horizon]

For question 8, take note of the fact that we have a monoprotic acid as well as a monobasic alkali. This means that the amount of acid (in mol) will be equal to the amount of alkali (in mol).

For question two, look at what H2SO4 does? It donates protons.

For Q8) I'm finding that the molar mass of the monoprotic acid is 18.06..which doesn't match up with any of the options. Have I done something wrong with my calculations?

Q10) A general question: When you are reading the concentration off a calibration curve, how accurate should you take the reading (that is, to how many sig figs?)

[charmanderp]

Q8) Aaah so it is. I'm awfully confused :/ HCl is exactly twice that molar mass, so maybe we've both missed a step....

However many sig figs the x-axis gives you?

[horizon]

When I find the value of the concentration on the x axis, it falls exactly in between two of the lines in the scale- e.g. 0.00245.
Should I round up or keep the half?

Q11) NaH(s) + H2O(l) → Na+(aq) + OH-(aq) + H2(g)
The half equation for the reduction is this:
Reduction: 2H2O(l) + 2e- → H2(g) + 2OH-

I haven't learnt how to write half equations where there are two "products" i.e. H2 and OH- at the end.  Could anyone explain to me why this is so?

[charmanderp]

Just guesstimate to the best of your ability. In terms of short answer questions, there's usually a reasonable allowance in terms of decimal places from reading graphs.

In water, NaH ionises to H-.

[Nobby]

Just guesstimate to the best of your ability. In terms of short answer questions, there's usually a reasonable allowance in terms of decimal places from reading graphs.

In water, NaH ionises to H-.

H^- + H₂O -> H₂ + OH^-

[charmanderp]

^Exactly.

[horizon]

Q12)
Concentrated sulfuric acid, H2SO4 (l), can be used to
A.   oxidise magnesium chloride to chlorine and magnesium metal
B.   reduce calcium oxide to calcium
C.   produce hydrogen chloride from sodium chloride
D.   produce carbon monoxide from sodium carbonate

Answer is C, but how though?

[Destiny]

A.  I think the chlorine bit is oxidised, and the magnesium is reduced, so I don't think it can be A because of the 'and'
D.  I think acid + metal carbonate = salt + water + Co2 so I don't think it can be D.
C.  I think my teacher said Cl- is a horrible base, but it is the conjugate base of HCl, so that's why I'd go with C.

[horizon]

Q13)

An important use of sulfuric acid is for the manufacture of superphosphate according to the equation
Ca3(PO4)2 (s) + 2 H2SO4 (aq) + 4 H2O (l) --> Ca2(PO4)2 (s) + 2 CaSO4.2H2O (s)
In this reaction sulfuric acid is acting as
A.   a dehydrating agent
B.   an oxidant
C.   a reductant
D.   an acid
Answer: D, how though?

Q14)

4.65 g of metal X formed an oxide with an empirical formula of XO3 and a mass of 8.94 g. What is the relative atomic mass of the metal X?
A.   55.9
B.   52.0
C.   27.0
D.   10.8

Answer: B, how would you work it out with looking at the options?

Q15)

Concentrated sulfuric acid is used to dry many gases. It cannot, however, be used to dry
A.   ammonia
B.   carbon dioxide
C.   sulfur dioxide
D.   oxygen

Answer: A, why?