Chemistry 101

[Mercedes Benzene]

1. Yes

2. I guess you COULD say that it's a solution of Calcium Oxide... but general conventions would say that a solution is named for its species, and NOT its precursor.

3. It's still a double displacement reaction, but as I said, the Ammonium Hydroxide decomposes... AFTER the double displacement reaction occurs. is not a stable species.

[Mohit Pandey]

In my book-

If water is heated, then its pH decreases,i.e becomes acidic.

The reason was given that the level of dissociation increases and thus, hydronium ions concentration increases.
But I think that concentration also increases with hydronium ions and thus the total pH remains same. Please tell me about it.

[modest]

Originally Posted by Mohit Pandey

In my book-

The reason was given that the level of dissociation increases and thus, hydronium ions concentration increases.
But I think that concentration also increases with hydronium ions and thus the total pH remains same. Please tell me about it.

You are correct that the hydronium and hydroxide ions dissociate equally. It would look like this:


The amount water dissociates depends on its dissociation constant or equilibrium constant. The equilibrium constant of water depends on its temperature. Basically it means that the greater the temperature the more dissociated ions there are in solution.

The reason this lowers the PH in hotter water is because PH is defined by the amount of hydronium ions and not hydroxide ions. The wikipedia article on PH is a good read on this.

I also would recomend the article on self-ionization of water which describes this process in more detail.

Originally Posted by wikipedia

Since the concentration of hydronium at SATP [Standard Ambient Temperature and Pressure] (approximately 25 °C) is 1.0×10−7mol/l, the pH of pure liquid water at this temperature is 7. Since Kw increases as temperature increases, hot water has a higher concentration of hydronium than cold water (and hence lower pH), but this does not mean it is more acidic, as the hydroxide concentration is also higher by the same amount.

Is this any help?

-modest

[Mohit Pandey]

One minute, does it mean that not every solution having lower pH is acidic in nature and vice versa?

[modest]

Originally Posted by Mohit Pandey

One minute, does it mean that not every solution having lower pH is acidic in nature and vice versa?

If you define acidity to mean a pH of less than 7 then hot water would both be slightly acidic and have a pH of slightly less than 7. This slight deviation from pH's definition (which relies on 25 Celsius ) is only important when you are doing something that accurately depends on acidity.

For instance, if you are mixing an acid and a base, let's say hydrochloric acid and sodium hydroxide:



and measuring the result with a pH meter and you need the hydrochloric acid to completely neutralize the sodium hydroxide and the solution isn't at 25 C then you will need to take into account the difference in pH from the standard definition.

Otherwise, you will be fine. Adding an acid to water means more in the water means more acidic means lower pH.

It's just important to know that the only reason hot water is more acidic is because our deffinition relies on the hydronium, not the hydroxide... . If we were using pOH, we would get the opposite result with hot water... .

-modest

[Mohit Pandey]

Otherwise, you will be fine. Adding an acid to water means more in the water means more acidic means lower pH.

But I have read that mixing an acid or base with water results in decrease in concentration of ions per unit volume. This is called dilution.

[Mohit Pandey]

The atomic of an element is expressed relative to isotope of carbon which has an exact value of 12. For example, the mass of oxygen- 16 is 15.995 u.
I wonder how these values of a particular element can be so close to mass number of that particular element. Mass number is the number of total protons and neutrons found in its nucleus. For example, Oxygen has a mass number of 16. Is it a mere coincidence or there any scientific reason behind it?
P.S how to put 12 above C in latex tag?

[UncleAl]

Mass number is the number of total protons and neutrons found in its nucleus.

Atomic mass is the mass of an atom given (C-12)/12 = 1 amu. It is what it is observed to be - the sum of its constituents' masses (protons, neutrons, electrons) less nuclear and electronic binding energies/c^2 and some very small quantum corrections (e.g., Lamb shift).

Look Up Nuclear Masses and Deformations
Exact Masses and Isotopic Abundances of the Elements - Alphabet
Chemistry::Isotope - Table of the isotopes exact mass data - search.cpan.org
links therein

[modest]

Originally Posted by Mohit Pandey

But I have read that mixing an acid or base with water results in decrease in concentration of ions per unit volume. This is called dilution.

Yeah, if you mix an acid with water then the water gains ions and the acid looses ions. The acid becomes more dilute and the water becomes more acidic. All depends on which reactant you're talking about. From the acid's perspective adding water makes the acid dilute with less hydronium per volume. From the water's perspective adding acid makes the water more acidic by increasing the concentration of hydronium.

Originally Posted by Mohit Pandey

The atomic of an element is expressed relative to isotope of carbon which has an exact value of 12. For example, the mass of oxygen- 16 is 15.995 u.
I wonder how these values of a particular element can be so close to mass number of that particular element.

Carbon-12 has six neutrons and six protons. Atomic mass is defined with C12. One atomic mass unit is 1/12 of carbon-12. However, if you look at the element on the periodic table you will see its atomic mass listed as 12.011. This is actually the relative atomic mass and it isn't exactly 12 because on earth most samples of carbon you run into have mostly carbon-12 but a little carbon-13 (which has 7 neutrons). When both isotopes are mixed the average amu is what you see listed. The same goes for oxygen and the other elements.

Check out:
Atomic mass - Wikipedia, the free encyclopedia

Originally Posted by Mohit Pandey

P.S how to put 12 above C in latex tag?

^{12}C will render as

-modest

[Mohit Pandey]

Carbon-12 has six neutrons and six protons. Atomic mass is defined with C12. One atomic mass unit is 1/12 of carbon-12. However, if you look at the element on the periodic table you will see its atomic mass listed as 12.011. This is actually the relative atomic mass and it isn't exactly 12 because on earth most samples of carbon you run into have mostly carbon-12 but a little carbon-13 (which has 7 neutrons). When both isotopes are mixed the average amu is what you see listed. The same goes for oxygen and the other elements.

OK. But I had asked why atomic mass, which is relative to mass of C-12 and mass number, which is found by counting the no.s of protons and neutrons, are so close?