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Some Basic Concepts of Chemistry — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistrySome Basic Concepts of Chemistry5 Marks
Question
If the density of methanol is $0.793 \mathrm{~kg} \mathrm{~L}^{-1}$, what is its volume needed for making 2.5 L of its 0.25 M solution?
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Answer

$\text { Molar mass of methanol }\left(\mathrm{CH}_3 \mathrm{OH}\right)=(1 \times 12)+(4 \times 1)+(1 \times 16)$
$=32 \mathrm{~g} \mathrm{~mol}^{-1}$
$=0.032 \mathrm{~kg} \mathrm{~mol}^{-1}$
Molarity of methanol solution $=\frac{0.793 \mathrm{~kg} \mathrm{~L}^{-1}}{0.032 \mathrm{~kg} \mathrm{~mol}^{-1}}$
$=24.78 \mathrm{~mol} \mathrm{~L}^{-1}$
(Since density is mass per unit volume)
Applying,
$\mathrm{M}_1 \mathrm{~V}_1=\mathrm{M}_2 \mathrm{~V}_2$
(Given solution) (Solution to be prepared)
$\left(24.78 \mathrm{~mol} \mathrm{~L}^{-1}\right) \mathrm{V}_1=(2.5 \mathrm{~L})\left(0.25 \mathrm{~mol} \mathrm{~L}^{-1}\right)$
$\mathrm{V}_1=0.0252 \mathrm{~L}$
$\mathrm{~V}_1=25.22 \mathrm{~mL}$

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