Some basic chemistry problems i have troube with

In summary, the conversation discussed three problems related to General Chemistry. The first problem involved finding the moles of Pb2+ in a given volume and molarity of Pb(NO2). The second problem discussed whether a reaction involving a solid in a hot bath was endothermic or exothermic. The third problem focused on finding the concentration of H+ and CN- in a solution with a known pH. Solutions and explanations were provided for each problem.
  • #1
nemzy
125
0
Hi, i am a 3rd year in General Chemistry, freshmen at a university, and i have a few problems that I've encountered along the way. These questions has been bugging me for a long time now and i really hope someone can fill in the holes and gaps in for me..anyways here they are

1) Let's say you have a .00035 L volume of .25 M Pb(NO2). What are the moles of Pb2+?

This is my guess on how to do this. Since molarity = moles/volume...molarity of pb2+ = moles of pb2+/.00035 But how would i know what the molarity of Pb2+ is?? Can i just use .25 M Pb(N02) instead? Is it the same thing or am i missing something here

2) Let's say you put a test tube which has a solid in a hot bath for 30 seconds. Then you shake and stir the test tube and the solid precipitates. Is this an endo or exo thermic reaction?

My guess is that its exo, because the surroundings are hot? But i am not sure, since the precipitate dissapears, wouldn't the equilibrium shift to the reactant side, meaning few products, which means endothermic??

3) Let's say you have a pH of 0.10 M HCN solution which is 5.2. What is the [H+] and [CN-].

I know that pH = -log[H+], so to find the [H+] i would just take the antilog, but then how would i find the [CN-]?
 
Chemistry news on Phys.org
  • #2
i thought you need the amount (moles) of Pb2+?
 
  • #3
1)

Definition of Molarity = (no of moles of solute)/(Volume of solution)

Now Pb(NO2) contains equal no of moles of Pb2+ and N02-

3) use charge conservation
 
  • #4
1) "Lets say you have a .00035 L volume of .25 M Pb(NO2). What are the moles of Pb2+?"

if that is the question then i think it wants you to find the number of moles instead of molarity.

moles of Lead II Nitrite = volumn x molarity = .00035 x .25 = 8.75 x 10^-5

But since the question asked for Plumbous II (Lead) which has a valence of two plus compared to the minus one valence of Nitrite, there are therefore one Lead II per molecule of Lead II Nitrite. This means there are the same number of moles of Lead II compared to Lead II Nitrite. i.e. 1 to 1 ratio.

:. moles of Lead II = 8.75 x 10^-5

2) "Lets say you put a test tube which has a solid in a hot bath for 30 seconds. Then you shake and stir the test tube and the solid precipitates. Is this an endo or exo thermic reaction?"

The forward reaction is endothermic. If this is what you mean:

solid dissolved (aq)---> heat ---> solid precipates (s)

Remember endothermic reaction means a reaction which occurs with absorption of heat.

The back reaction is exothermic since heat must be released to the environment for the solid to dissolve again. This can be done by leaving the test tube in an environment with lower temperature than itself so that entropy is favoured for the direction of heat transfer.

3) "Lets say you have a pH of 0.10 M HCN solution which is 5.2. What is the [H+] and [CN-]."

By the Arrhenius acid and base model: Acids are chemicals which ionizes in water to produce hydrogen ions. i.e. HCN ---> Hydrogen ion + Cyanide ion.

This ionization, in this case described in the question produces equal amounts of H and CN since in the molecule of HCN the ratio of H:CN is 1:1.

Therefore if you found the moles of H, you found the mole of CN. :smile:

Hopefully this helps.
 
Last edited:

1. How can I balance chemical equations?

Chemical equations must be balanced in order to follow the law of conservation of mass. To balance an equation, you must ensure that the number of atoms of each element on the reactant side is equal to the number of atoms on the product side. You can balance equations by adjusting coefficients, not subscripts.

2. What is the difference between an element and a compound?

An element is a pure substance that cannot be broken down into simpler substances by chemical means. It is made up of only one type of atom. A compound is a substance made up of two or more elements chemically combined in a fixed ratio. It can be broken down into its individual elements through chemical reactions.

3. How do I calculate molar mass?

Molar mass is the mass of one mole of a substance, measured in grams. To calculate molar mass, you must first determine the atomic mass of each element in the compound by referring to the periodic table. Then, multiply the atomic mass by the number of atoms of each element in the compound. Finally, add all of the individual masses together to get the total molar mass.

4. What is the difference between an acid and a base?

Acids are substances that donate hydrogen ions (H+) in a chemical reaction, while bases are substances that accept hydrogen ions. Acids have a pH less than 7 and taste sour, while bases have a pH greater than 7 and taste bitter. Acids and bases can neutralize each other to form salts and water.

5. What is the difference between an exothermic and endothermic reaction?

An exothermic reaction releases energy in the form of heat, while an endothermic reaction absorbs energy. In an exothermic reaction, the products have less energy than the reactants, resulting in a negative change in enthalpy (ΔH). In an endothermic reaction, the products have more energy than the reactants, resulting in a positive ΔH.

Similar threads

Replies
16
Views
2K
Replies
2
Views
4K
Replies
6
Views
4K
  • Biology and Chemistry Homework Help
Replies
4
Views
3K
  • Biology and Chemistry Homework Help
Replies
4
Views
3K
  • Biology and Chemistry Homework Help
Replies
2
Views
9K
  • Biology and Chemistry Homework Help
Replies
2
Views
7K
  • Biology and Chemistry Homework Help
Replies
4
Views
10K
  • Chemistry
Replies
2
Views
3K
Replies
2
Views
5K
Back
Top