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dsp73
3 years ago
7

When a strong acid is titrated with a strong base using phenolphthalein, the color changes suddenly at the end point. The color

can be switched back and forth by addition of only a single drop of acid or base. The reason for the abruptness of this color change is that?
Chemistry
1 answer:
larisa86 [58]3 years ago
4 0

Answer:

Keto - enol tautomerism

Explanation:

Phenolphthalein is an organic molecule (formula: C20H14O4), which is used in to determine the final volume in an acid-base titration, this is, as the molecule changes from one to another color, depending on the Ph of the solution (in acids solution, the molecule remains colorless, while in basics solutions it remains pink)  :

The molecule of phenolphthalein is a week acid, which losses a proton (H+) when it´s in solution: the undissociated molecule is colorless, while the correspondent anion (without a H+) is pink. This may be simplified with the following reaction:

H₃In⁺ ⇄ H₂In (colorless) ⇄ In⁻² (pink) ⇄ In(OH) ⁻³

- When adding a base (for example, NaOH) to phenolphthalein (initially, a colorless solution), molecule losses one H⁺ and we obtain In⁻², which is pink

- When adding an acid, the original structured is recovered, so color turns from pink to colorless

This change color is not only explained with adding an acid or a base, but also with phenolphthalein structure, that leads to a keto-enol tautomerism: as molecule has 2 hydroxyl groups (- OH) and 1 carbonyl group (C = O), compounds with this structure have an equilibrium between both groups:

R1 = C H– OH ⇄  R1 – C = O

With this change, phenolphthalein structure changes and hence, color solution changes as well

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How many nitrogen molecules are in 1 liter of nitrogen gas at STP?

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Pete Gannett

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Ph.D. Chemistry, University of Wisconsin-Madison, (1982)2y

Seems to be an ideal gas law question. The relevant equation is:

PV = nRT

where P is the pressure in atmospheres, V is the volume in liters, n is the number of moles of gas, R is the gas constant (0.082 atm-L/mole-deg K), and T is temperature in Kelvins. STP means standard temperature and pressure and this is taken as 1 atm and 0º C or 273 K.

To calculate the number of molecules we will use the constant 6.023 * 10^23 molecules/mole and, therefore, we will need to know the number of moles (n). So, first we’ll rearrange the gas law equation, isolating ’n’ and then put the numbers in.

n = PV/RT = 1 * 1 / (0.082)(273) = 0.0447 moles

So, to calculate the number of molecules, multiple this by the number of molecules in a mole and you get:

# molecules of nitrogen in 1 Liter at STP = 6.023 * 10^23 molecules/mole * 0.0447 moles = 2.6905 * 10^22 molecules

Note, it does not matter what the gas is.

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H3PO4 + Ca(OH)2 → Ca(H2PO4)2 + H2O
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Given question is incomplete. The complete question is as follows.

Balance the following equation:

H_3PO_4 + Ca(OH)_2 \rightarrow Ca(H_2PO_4)_2 + H_2O

Answer: The balanced chemical equation is as follows.

2H_3PO_4 + Ca(OH)_2 \rightarrow Ca(H_2PO_4)_2 + 2H_2O

Explanation:

When a chemical equation contains same number of atoms on both reactant and product side then this equation is known as balanced equation.

For example, H_3PO_4 + Ca(OH)_2 \rightarrow Ca(H_2PO_4)_2 + H_2O

Number of atoms on reactant side:

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Number of atoms on product side:

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In order to balance this equation, we will multiply H_3PO_4 by 2 on reactant side and we will multiply H_2O by 2 on product side. Hence, the balanced chemical equation is as follows.

2H_3PO_4 + Ca(OH)_2 \rightarrow Ca(H_2PO_4)_2 + 2H_2O

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