HSC Chemistry Question Thread

[jakesilove]

hi!
how do you work out the concentration of an acid given the pH of a substance?
for example,
A solution of HNO3 has a pH of 2.05. Calculate the concentration of the acid.

thank you!

Hey! We have a nice little formula for this, on your Chemistry formula sheet. It says,



Remember that, if



then



So, we can rearrange the above given equation to read



ie. 10 to the power of the negative pH will give you the concentration! In this case,

[bananna]

Hey! We have a nice little formula for this, on your Chemistry formula sheet. It says,



Remember that, if



then



So, we can rearrange the above given equation to read



ie. 10 to the power of the negative pH will give you the concentration! In this case,

thank you!
forgot about that formula
also,
how do calculate the pH of 0.055M Ba(OH)2?
do you assume the 2xOH molecules are included in the molar concentration? (if that makes sense)

thank you!

[jakesilove]

thank you!
forgot about that formula
also,
how do calculate the pH of 0.055M Ba(OH)2?
do you assume the 2xOH molecules are included in the molar concentration? (if that makes sense)

thank you!

That's exactly what you do! So,



As the concentration of OH will be twice the concentration of the base.

[Kekemato_BAP]

Hi, are there any previous guides on how to properly draw Lewis dot diagrams? I always lose a mark for non-proper scientific drawing of it, thanks 

[ellipse]

Need help on this questions please
Students measured the mass change in a flask containing glucose solution and yeast. The initial mass of the
flask and contents was 428.1 g and the mass of the flask and contents after a week was 390.1 g.
What is the mass of alkanol produced?

[jakesilove]

Hi, are there any previous guides on how to properly draw Lewis dot diagrams? I always lose a mark for non-proper scientific drawing of it, thanks 

Hey! This site here has heaps of examples! Let me know if there is a specific question that you're struggling with.

[jakesilove]

Need help on this questions please
Students measured the mass change in a flask containing glucose solution and yeast. The initial mass of the
flask and contents was 428.1 g and the mass of the flask and contents after a week was 390.1 g.
What is the mass of alkanol produced?

Hey! First, let's look at the chemical formula



So, where would mass be lost? In the Carbon dioxide gas! We can assume that all mass change (ie. 38g) was a result of lost Carbon dioxide. So,



Well, if there are 0.863 moles of Carbon dioxide, there will also be 0.863 moles of the Alkanol (Ethanol). Therefore

[ellipse]

Hey! First, let's look at the chemical formula



So, where would mass be lost? In the Carbon dioxide gas! We can assume that all mass change (ie. 38g) was a result of lost Carbon dioxide. So,




Well, if there are 0.863 moles of Carbon dioxide, there will also be 0.863 moles of the Alkanol (Ethanol). Therefore

Hey Jake! I got the same mass as well, but the answer says that it is 19.9g. So is 19.9g wrong?

[Kekemato_BAP]

Hey Jake! I got the same mass as well, but the answer says that it is 19.9g. So is 19.9g wrong?

Yeah, it should be the 39.7g. The 19.9 value is half of the 39.7 value so the textbook may have had the wrong stoichiometry ratio

[Kekemato_BAP]

Would you need to know how to calculate pH of a non-neutral salt?
Eg. 20mL of 1M citric acid + 25mL of 0.5M sodium hydroxide? How do you calculate H+ concentration of a weak acid when it doesn't ionise fully? Thanks

[Kle123]

Why do we need a secondary solution? Is its concentration supposed to be more accurate than the primary solution? How can measure something to be more accurate than the solution it was used to create it?

These are thoughts that pop up when i learned titration. Teacher never explained it. Could someone please help me? THANKS

[kiwiberry]

After you distill an ester, how would you confirm its purity?

[bsdfjnlkasn]

After you distill an ester, how would you confirm its purity?

I'm guessing you would weigh out the ester and continue to fractionally distill your sample until the weight plateaus - this would indicate that all impurities have filtered out. Mind you, I am just applying general theory about obtaining pure samples but since esters are liquids it maybe a bit different when discussing impurities and how to get rid of them.

[Kekemato_BAP]

Can someone explain the uses of Ka, Kb, Kw, etc? What are they used for? Thanks

[Bubbly_bluey]

Hey! I have a question:
 A student predicts 100mL of 10mol/L NaOH mixed with 100mL H₂SO₄ of 10mol/L with release about 114kJ. Is this prediction true?
Thank you