Amolecule has one carbon-carbon double bond and four monovalent atoms or groups. how many different cis-trans isomers exist for this molecule?

There can be two cases

Case 1:

When a molecule has one carbon-carbon double bond and four monovalent atoms or groups in which two atoms on same carbon are different, there will be 2 isomers, one cis and one trans.

Case 2:

When a molecule has one carbon-carbon double bond and four monovalent atoms or groups where both atoms attached to the same carbon are same, there will be no geometrical isomerism.

As based on priority, there will be only two orientations possible , in which same priority atoms are on the same side that is cis and if they are on the opposite side that is trans.

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.

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.