Prelim Chem Thread (1 Viewer)

[30june2016]

Determine the molecular formula and homologous series to which straight chain hydrocarbons with the following molecular weights could belong. Values for the molecular weights are rounded.

a) 28
b) 42
c) 72

We know that the molar mass of Carbon is roughly 12 and Hydrogen is roughly 1

so

a) 28
Carbon + Hydrogen = 12 + 1 = 13
28/13 = 2 remainder 2
therefore there are 2 carbons and 2+2 hydrogens
so the molecular formula is C2H4 and is thus an Alkene (formula: CnH2n)
b) 42
do it the same way to get C3H6
therefore it is also an Alkene
c) 72
C5H12
therefore it is an Alkane (CnH2n+2)

 

[eyeseeyou]

Does the pressure a substance exerts on the container increase as it goes from solid to liquid to gas? Or is it the other way around?

Are you talking about the metals module?

 

[drsabz101]

Hey Guys! I would really appreciate if someone could help solve this style question asap.

q. what is the molarity of:
silver ion in a solution , 50ml of which produced 1.87g silver chloride when excess sodium chloride was added to it?

 

[Fizzy_Cyst]

Hey Guys! I would really appreciate if someone could help solve this style question asap.

q. what is the molarity of:
silver ion in a solution , 50ml of which produced 1.87g silver chloride when excess sodium chloride was added to it?

- Write balanced eqn for formation of ppt
- Find number of moles of ppt
- Find number of miles of ion (from balanced eqn)
- find concentration (c=n/v)

 

[eyeseeyou]

- Write balanced eqn for formation of ppt
- Find number of moles of ppt
- Find number of miles of ion (from balanced eqn)
- find concentration (c=n/v)

What does this stand for?

 

[BlueGas]

What does this stand for?

Precipitate

 

[eyeseeyou]

Precipitate

Thanks

 

[drsabz101]

Thanks. Do you know how to write the equation , is it Ag? + NaCl-> AgCl(s) + ? I t just says a silver ion

 

[jathu123]

Thanks. Do you know how to write the equation , is it Ag? + NaCl-> AgCl(s) + ? I t just says a silver ion

We did this prac,
Its AgNO3 (aq) + NaCl (aq) -->AgCl (s) + NaNO3 (aq)

You can decompose this AgCl with light;
2AgCl (s) --> 2Ag (s) + Cl2 (g)

 

[eyeseeyou]

1. A student dissolves 5.3g of anhydrous sodium carbonate (Na2CO3) in water and then adds more water to make the volume up to 125 ml
a)why did the student first dissolve it on water and then make up the final volume by adding water
b)Why did the student use Anhydrous Na2CO3?

2. Calculate how many grams of solute are in 500mL of 0.5mol L^-1 sodium cholride (nacl) solution

 

[eyeseeyou]

Does the pressure a substance exerts on the container increase as it goes from solid to liquid to gas? Or is it the other way around?

That way is correct.

Pressure only ever relates to the quantity of gaseous substances present in a (generally speaking, closed) system. Liquids and solids are not counted, so if gas is converted into liquid/solid we go the other way and decrease pressure.

 

[Snowflek]

Hello, i need some assistance with this question. So i asked my teacher and she said use the c1v1=c2v2 method. How do we know if we use this formula? Cause for some reason i was thinking about the c=n/v
13. A student takes 10 mL of a 0.45M solution and adds 990 mL.

What will the new concentration be?
Answer is 0.0045M. Can someone show the full working out for me thank you

 

[InteGrand]

Hello, i need some assistance with this question. So i asked my teacher and she said use the c1v1=c2v2 method. How do we know if we use this formula? Cause for some reason i was thinking about the c=n/v
13. A student takes 10 mL of a 0.45M solution and adds 990 mL.

What will the new concentration be?
Answer is 0.0045M. Can someone show the full working out for me thank you

 

[Snowflek]

Thank you!

 

[Snowflek]

Can someone help me with this question?
Which process requires the largest energy input for a given mass of water?
A) Melting
B) Heating to boiling point
C) vaporisation to steam
D)electrolysis
I was thinking B but not too sure D:

Which of the following is an element that shows valencies of 3 and 5?
a) carbon
b) sulfur
C)phosphorus
D)oxygen
Can someone explain this question too me.. I dont get it on how it can be both 3 and 5 valency

 

[Fizzy_Cyst]

Can someone help me with this question?
Which process requires the largest energy input for a given mass of water?
A) Melting
B) Heating to boiling point
C) vaporisation to steam
D)electrolysis
I was thinking B but not too sure D:

Which of the following is an element that shows valencies of 3 and 5?
a) carbon
b) sulfur
C)phosphorus
D)oxygen
Can someone explain this question too me.. I dont get it on how it can be both 3 and 5 valency

First question:
A, B, C are all physical changes (less energy needed), D is a chemical change (decomposing H2O into H2 and O2 -- more energy required for this)

Second Question:
Comes back to sub shells. Phosphorus has 5 valence electrons, 2 in the 3s sub shell and 3 in the 3p sub shell.

It has a valency of 5 when all of its 3rd shell electrons are used in bonding, but 3 when only the electrons from the 3p sub shell are used.

 

[Snowflek]

Thank you so much!

 

[Snowflek]

A 10.0mL volume of 0.10mol CaCl2 solution is diluted to 1.0L using distilled water.
What is the concentration of calcium ions, in mol L, in the resulting solution?
a) 0.001
b)0.002
c)0.02
D)0.05
I used C1v2=c2v2 and i did
0.10x10/990 =0.00101010101 so i got a, is it correct?
Sorry if its a dumb question, math is just my weak point and it seems like i cant understand majority of the questions that includes math D:

 

[InteGrand]

A 10.0mL volume of 0.10mol CaCl2 solution is diluted to 1.0L using distilled water.
What is the concentration of calcium ions, in mol L, in the resulting solution?
a) 0.001
b)0.002
c)0.02
D)0.05
I used C1v2=c2v2 and i did
0.10x10/990 =0.00101010101 so i got a, is it correct?
Sorry if its a dumb question, math is just my weak point and it seems like i cant understand majority of the questions that includes math D:

 

[InteGrand]

It's easier to just see it intuitively since the numbers are easy to calculate mentally here. The new volume is 100 times the old one, so the concentration must become 100 times smaller. The original concentration was 0.1 mol/L, so the new one is one-hundredth of this, i.e. 0.001 mol/L (shift the decimal point in 0.1 two spaces left to make the number 100 times smaller).