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The answer is: Charle's Law.
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Why?</h2>
The law that states that the volume and absolute temperature of a fixed quantity of gas (ideal gas) are proportional under constant pressure is the Charle's Law, also known as the law of volumes.
The law describes how a gas kept under constant pressure tends to expand when the temperature increases and it's described by the following equation:

Where,

Also, to describe the relationship between two differents volumes at different temperatures, we have:

Where,

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Explanation:tr
a) Molar mass of HF = 20 g/mol
Atomic mass of hydrogen = 1 g/mol
Atomic mass of fluorine = 19 g/mol
Percentage of an element in a compound:

Percentage of fluorine:

Percentage of hydrogen:

b) Mass of hydrogen in 50 grams of HF sample.
Moles of HF = 
1 mole of HF has 1 mole of hydrogen atom.
Then 2.5 moles of HF will have:
of hydrogen atom.
Mass of 2.5 moles of hydrogen atom:
1 g/mol × 2.5 mol = 2.5 g
2.5 grams of hydrogen would be present in a 50 g sample of this compound.
c) As we solved in part (a) that HF molecules has 5% of hydrogen by mass.
Then mass of hydrogen in 50 grams of HF compound we will have :
5% of 50 grams of HF = 
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Answer:
single replacement
Explanation:
If it has one plus sign it is decomposition or synthesis
however if it has two plus signs then it is single or double replacement
Answer is: d). 1-propanol. Because between two molecules of 1-propanol can for form hydrogen bond, between hydrogen and oxygen in hydroxy group.
1) Chemical formula for propanal is CH₃-CH₂-CH=O. Propanal is a saturated three carbon aldehyde (have a carbonyl center).
2) Chemical formula for propane is CH₃-CH₂-CH₃. Propane is a three carbon alkane (acyclic saturated hydrocarbon).
3) Chemical formula for propanone is (CH₃)₂-C=O. Propanone or acetone is he simplest and smallest ketone.
4) Chemical formula for propanol is CH₃-CH₂-CH₂-OH. 1-propanol is a primary alcohol.
Hydrogen bond is
an electrostatic attraction between two polar groups that occurs when
a hydrogen atom (H), covalently bound to a
highly electronegative atom such as flourine (F), oxygen (O) and
nitrogen (N) atoms.