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# $1g$ of a sample of brass on reacting with excess $HCl$ produces 120ml of ${H_2}$ ​gas at STP. The percentage Zn in this sample of brass is: (Atomic weight of Zn-=65.5g)(A) $32\%$ (B) $35\%$ (C) $38\%$ (D) $40\%$

Last updated date: 12th Aug 2024
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Hint- In this problem we will come across various concepts like moles, mole concept and stoichiometry concept. We will study all these concepts in detail followed by the required reactions and formulas which will further help us in approaching our solution.

Complete step by step solution:
> Moles: For particles $1\,mol = 6.022 \times {10^{23}}$. The International System of Units (SI)recognizes the mole (symbol: mol) as the unit of measurement for amount of substance. If any substance $6.02214076 \times {10^{23}}$ particle is present then that substance quantity is said to be one. The particles, which are present may be atoms, molecules, ions, or electrons.
The definition came in order in November 2018. Mole was then recognised as one of the seven SI base units. This definition is the revised definition of the previous definition that coined it as the number of atoms in $12\,grams$ of carbon$12\left( {^{12}C} \right)$ , an isotope of carbon.

> Avogadro’s number $\left( {6.022 \times {{10}^{23}}} \right)$ this number is named as Avogadro’s number. It was founded by Amadeo Avogadro. He said at particular pressure and temperature the volume of the gas will rise if there is rise in the number of atoms or molecules. The nature of the gas does not play a role. It is symbolized by $\left( {{N_A}} \right)$ this number is an absolute number. As there are $6.022 \times {10^{23}}$ basic units in 1 mole. TheAvogadro’s number can also be written as $6.022 \times {10^{23}}$ mol-1 .
This number concept is more likely to the concept of a dozen or a gross. A dozen molecules is $12$ molecules. A gross of molecules is $144$ molecules. Avogadro’s number is$6.022 \times {10^{23}}$ molecules.

> Stoichiometry concept: It is made up of two words which means elements and measurement, so finally stoichiometry is the measure of elements. Stoichiometry makes relation between reactants and/or products whenever a chemical reaction is concerned. It deals with all types of connection between element and its measurement.
$Zn + 2HCl \to ZnC{l_2} + {H_2}$
Moles of ${H_2}$ released =$\dfrac{{120}}{{22400}} = 0.0053$
Therefore, Moles of $Zn$$= \dfrac{{0.0053}}{{20}} = 5.3mole$
Therefore % of $Zn$$= \dfrac{{5.3 \times {{10}^{ - 3}} \times 65.5}}{1} \times 100$
$\Rightarrow 35.7$
Hence, we can come to a result that $35.7$ that is option B is right.

Hence, the correct option is B

Note - In commercial world, an alloy of $Zn$ and $Cu$ which is known as Brass, reacts with hydrochloric acid and the reaction is shown below –
$Zn(s)\ + 2HCl(aq)\, \to \,ZnC{l_2}(aq) + {H_2}(g)$
In this, $Cu$ does not react with $HCl$.
When $0.5065$g of brass reacts with excess HCl, $0.0985$g of $ZnC{l_2}$ is isolated.