Question

A mixture of aluminium and zinc weighing $1.67grams$ was completely dissolved in acid and $1.69litres$ of hydrogen gas was evolved. The hydrogen gas was measured at STP conditions .what was the mass of aluminium in original mixture

Answer 1

STP conditions are those in which temperature and pressure both have standard value . when the aluminium and zinc mixture is mixed with acid it evolves hydrogen and aluminium ions and zinc ions . we need to write the equation and balance it to check the amount of hydrogen evolved.

Complete step by step answer:
Let the aluminium contain in the mixture be $x$
Zinc present in the mixture be $1.67 - x$
The mass of aluminium is $27g/mol$
The mass of zinc is $65.4g/mol$
The number of moles that may present be $\dfrac{x}{{27}}g/mol$
The number of mole present of zinc is $\dfrac{{1.67 - x}}{{65.4}}g/mol$
Now according to the question the reactions which are taking place are as follows :
$2Al + 6{H^ + } \to 2A{l^{3 + }} + 3{H_2}$
$Zn + 2{H^ + } \to Z{n^{2 + }} + {H_2}$
Volume of hydrogen obtained from aluminium is $\dfrac{{3 \times 22.4 \times x}}{{2 \times 27}}........\left( 1 \right)$
Volume of hydrogen obtained from zinc is $\dfrac{{22.4 \times 1.67 - x}}{{65.4}}......\left( 2 \right)$
Now adding the two equation
$\dfrac{{3 \times 22.4 \times x}}{{2 \times 27}} + \dfrac{{22.4 \times 1.67 - x}}{{65.4}} = 1.69$
$\Rightarrow$ $x = 1.248g$
Therefore percentage of aluminium is $\dfrac{{1.248}}{{1.67}} \times 100 = 75\%$

Additional information : Volume is considered standard conditions because volume is a function of both temperature and pressure. Which means it is necessary to state the conditions like pressure or temperature .Otherwise it may create error in the calculations . There is a term called NTP , the major difference between STP and NTP is NTP uses ${20^ \circ }C$ as temperature and $101.325kPa$.

Note:
In STP one mole contains $22.4litres$. In the laboratory we generally use standard ambient temperature. In the STP system the pressure is $760torr$ and temperature is $273K$. The national institute of standards and technology which defines a standard temperature is ${0^ \circ }$ and standard pressure is $1atm$. In the case of IUPAC they wrote that the standard temperature is ${0^ \circ }$ and standard pressure is $1bar$.