In the symbol 4 2he, the superscript 4 is the for helium and the subscript 2 is the for helium.
a. atomic number; mass number
b. mass number; atomic number
c. atomic number; atomic mass
d. atomic mass; atomic number

Answer is: b. mass number; atomic number.

The superscript is mass number and the subscript is atomic number.

Atomic number(Z) is the number of protons, which is characteristic of a chemical element..  

Mass number (A) is the total number of protons and neutrons in a nucleus.

Helium (symbol: He) is a chemical element with two protons, two electrons and two neutrons.

Option (b) is the correct answer.

Explanation:

Mass number means the total number of protons and total number of neutrons present in an atom. Mass number is written at the top left of the symbol of atom.

Whereas atomic number means the total number of protons present in the atom. Atomic number is written at the bottom left of the symbol of atom.

For example, in , the number 12 denotes mass number of carbon atom whereas 6 denotes the atomic number of carbon atom.

Thus, we can conclude that in the symbol , the superscript 4 is the mass number for helium and the subscript 2 is the atomic number for helium.

a mercury barometer is a device that uses the physical properties of mercury in a tube to measure barometric pressure. mercury barometers are used for : -

1. measurement of atmospheric pressure  

2. determining the altitude of a place

3. weather forecasting

4. calibration and checking of aneroid barometers

5. application in fluid mechanics

6. measurement of pressure in aircrafts

7. surface weather analysis

8. preparation of barographs

9. usage in physical sciences like physics, astronomy & chemistry

10. preparation of aircraft altimeters

characteristics of mercury barometer : -

1.used to define and measure atmospheric pressure.

2.on the average at sea level the column of mercury rises to a height of about 760 mm.

3.this quantity is equal to 1 atmosphere.

4.it is also known as standard atmospheric pressure.

5.a barometer in which the weight of a column of mercury in a glass tube with a sealed top is balanced against that of the atmosphere pressing on an exposed cistern of mercury at the base of the mercury column, the height of the column varying with atmospheric pressure.

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.