Answer:
A. 3 x 10-12
Explanation:
Generally, the parts-per notation is used to represent very small concentration without any specific unit.
Parts per million is notated as 
Parts per billion is notated as 
Parts per trillion is notated as 
Hence, 3 parts per trillion will be written as 3 x 
The correct option is A.
7.91 g/ml is the density of the iron piece of 28.5 gms.
Explanation:
The density of a substance is defined as the volume it occupies. It tells the matter present in a substance.
The density is mass per unit volume and is denoted by p.
The formula for density is given by:
density (p) = 
Data given is :
mass= 28.5 grams
V1 = 45.5 ml
V2= 49.1 ml
The initial volume of water was 45.5 ml, when iron piece of 28.5 grams was added the final volume was 49.1 ml.
Putting the values in the equation of density
p = 
p = 7.91 g/ml
Since iron is a dense material it will occupy less volume
Remember, look at the coefficients in the balanced equation! Here are three mole ratios:
1 mole<span> N2 / </span>3 moles<span> H2.</span>
3 moles<span> H2 / 2 moles NH3.
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Answer:

The air in the atmosphere consists of nitrogen, oxygen, which is the life-sustaining substance for animals and humans, carbon dioxide, water vapour and small amounts of other elements (argon, neon, etc.). Higher in the atmosphere air also contains ozone, helium and hydrogen.