Question

The atomic weights of two elements A and B are 40 and 80 respectively. If x g of A contains y atoms, how many atoms are present in 2x g of B?
A.$\dfrac{y}{2}$
B.$\dfrac{y}{4}$
C.$y$
D.2$y$

Answer 1

First find out the no of moles for both elements A and B. Then find out the no. of atoms by multiplying the moles with Avogadro's number. Then substitute y for the atoms of element A and find the atoms for element B.

Formula used: No. Of Moles = Given Weight/Atomic Weight

Complete Step By Step Solution:
Atomic weight for $A=\text{ }40$
So, for x g of A, the no. of moles = $\dfrac{x}{40}$
No. of atoms in one mole = $6.023\times {{10}^{23}}$ atoms i.e. equal to Avogadro no.
So, no. of atoms for moles of $$A\text{ }=$$ $$\dfrac{x}{40}\times 6.023\times {{10}^{23}}$$ atoms Given x g of A contains y atoms i.e. $$\dfrac{x}{40}\times 6.023\times {{10}^{23}}$$= y Now, atomic weight for B= 80 No. of moles for B= $$\dfrac{2x}{80}$$ So, no. of atoms in 2x g of B= $$\dfrac{2x}{80}\times $6.023\times {{10}^{23}}$=$$\dfrac{x}{40}$\times$6.023\times {{10}^{23}}$=y So, no. of atoms present in$2x\text{ }g\text{ }of\text{ }B=\text{ }y$$

Therefore, the correct option is (C).

Additional Information:
Mole: It is the unit of measurement for the amount of substance in the International System of Units (SI). A mole of a substance or a mole of particles is defined as exactly 6.02214076×1023 particles, which may be atoms, molecules, ions, or electrons.
Atomic Mass: The atomic mass of a single atom is simply its total mass and is typically expressed in atomic mass units or amu. For any given isotope, the sum of the numbers of protons and neutrons in the nucleus is called the mass number. This is because each proton and each neutron weigh one atomic mass unit (amu). By adding together the number of protons and neutrons and multiplying by 1 amu, we can calculate the mass of the atom.
Atoms of each element contain a characteristic number of protons. In fact, the number of protons determines what atom we are looking at (e.g., all atoms with six protons are carbon atoms); the number of protons in an atom is called the atomic number. If you want to calculate how many neutrons an atom has, you can simply subtract the number of protons, or atomic number, from the mass number.
Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Many elements—such as carbon, potassium, and uranium—have multiple naturally occurring isotopes. A neutral atom of Carbon-12 contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 (six protons plus six neutrons). Neutral carbon-14 contains six protons, eight neutrons, and six electrons; its mass number is 14 (six protons plus eight neutrons). These two alternate forms of carbon are isotopes.

Note: Study the concepts of moles, atomic masses and atomic no. One can study the periodic table for better understanding of atomic masses and atomic no. Read about Isotopes, Isotones along with their examples. Learn how to find out the moles and the atoms contained in them. The existence and importance of Avogadro no.