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A 3.4 g sample of an unknown monoprotic organic acid composed of C,H, and O is burned in air to produce 8.58 grams of carbon dio

Pavlova-9 [17]

Answer:

$C_7H_6O_2$

Explanation:

Hello there!

In this case, we can divide the problem in three stages: (1) determine the empirical formula with the combustion analysis, (2) compute the molar mass of acid via the moles of the acid in the neutralization and (3) determine the molecular formula.

(1) In this case, since 8.58 g of carbon dioxide are released, we can first compute the moles of carbon in the compound:

$n_C=8.58gCO_2*\frac{1molCO_2}{44.01gCO_2}*\frac{1molC}{1molCO_2}=0.195molC$

And the moles of hydrogen due to the produced 1.50 grams of water:

$n_H=1.50gH_2O*\frac{1molH_2O}{18.02gH_2O}*\frac{2molH}{1molH_2O} =0.166molH$

Next, to compute the mass and moles of oxygen, we need to use the initial 3.4 g of the acid:

$m_O=3.4g-0.195molC*\frac{12.01gC}{1molC}-0.166molH*\frac{1.01gH}{1molH} =0.89gO\\\\n_O=0.89gO*\frac{1molO}{16.0gO}=0.0556molO$

Thus, the subscripts in the empirical formula are:

$C=\frac{0.195}{0.0556}=3.5 \\\\H=\frac{0.166}{0.0556}=3\\\\O=\frac{0.0556}{0.0556}=1\\\\C_7H_6O_2$

As they cannot be fractions.

(2) In this case, since the acid is monoprotic, we can compute the moles by multiplying the concentration and volume of KOH:

$n_{KOH}=0.279L*0.1mol/L\\\\n_{KOH}=0.0279mol$

Which are equal to the moles of the acid:

$n_{acid}=0.0279mol$

And the molar mass:

$MM_{acid}=\frac{3.4g}{0.0279mol} =121.86g/mol$

(3) Finally, since the molar mass of the empirical formula is:

7*12.01 + 6*1.01 + 2*16.00 = 122.13 g/mol

Thus, since the ratio of molar masses is 122.86/122.13 = 1, we infer that the empirical formula equals the molecular one:

$C_7H_6O_2$

Best regards!

8 0

3 years ago

What are the limitations of litmus paper and Phenolphthalein indicators? Name to other indicators that can be used that do not h

jenyasd209 [6]

Answer:

Here's what I find.

Explanation:

An indicator is usually is a weak acid in which the acid and base forms have different colours. Most indicators change colour over a narrow pH range.

(a) Litmus

Litmus is red in acid (< pH 5) and blue in base (> pH 8).

This is a rather wide pH range, so litmus is not much good in titrations.

However, the range is which it changes colour includes pH 7 (neutral), so it is good for distinguishing between acids and bases.

(b) Phenolphthalein

Phenolphthalein is colourless in acid (< pH 8.3) and red in base (> pH 10).

This is a narrow pH range, so phenolphthalein is good for titrating acids with strong bases..

However, it can't distinguish between acids and weakly basic solutions.

It would be colourless in a strongly acid solution with pH =1 and in a basic solution with pH = 8.

(c) Other indicators

Other acid-base indicators have the general limitations as phenolphthalein. Most of them have a small pH range, so they are useful in acid-base titrations.

The only one that could serve as a general acid-base indicator is bromothymol blue, which has a pH range of 6.0 to 7.6.