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atroni [7]
3 years ago
9

Why a “control” is important in a case-control study of a disease?

Chemistry
2 answers:
nalin [4]3 years ago
8 0
To assess whether an association between a disease and a specific risk factor exists or not. Starting with a group of people with illness and working back to see if the cause could be a potential risk factor. In testing a hypothesis, this may be the first step.
bagirrra123 [75]3 years ago
6 0

Answer:

Explanation:

Purpose of Case Control Studies

To determine whether or not an association exists between a disease and a particular risk factor. To start with a group of people with disease and work back to see whether a possible risk factor may be the cause. It may be the first step in testing a hypothesis.

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Na2CrO4 + PbCL2<br> Formula in double replacement
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Na₂CrO₄ + PbCl₂ → PbCrO₄ + 2 NaCl

<u>Explanation:</u>

In a double displacement reaction, the reactants which are involved in the reaction exchanging their ions thereby produces 2 new compounds. Here sodium chromate and lead chloride are undergoing double displacement reaction, the ions exchanges their position there by forming sodium chloride and lead chromate. So the double displacement reaction is given as,

Na₂CrO₄ + PbCl₂ → PbCrO₄ + 2 NaCl

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Approximately how much of the world’s oil and natural gas reserves are believed to be in the arctic?
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In 2009, the US Geological Survey estimated the Arctic may be home to 30% of the planet's natural gas reserves and 13% of oil.
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Sodium hydroxide (NaOH) and hydrochloric acid (HCl) combine to make table salt (NaCl) and water (H2O). Which equation is balance
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The equation that is balanced among the given options is the first equation NaOH + HCl → NaCl + H2O

From the question,

We are to determine which of the options gives a balanced equation for the reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl) to make table salt (NaCl) and water (H₂O)

To do this, we will check which of the given equations is balanced, by checking the equations one after the other.

First, we will define what is meant by balanced equation.

Balanced equations are those whose coefficients result in equal numbers of atoms for each element in the reactants and products

  • For NaOH + HCl → NaCl + H2O

On the reactants side

Na = 1

O = 1

H = 2

Cl = 1

On the products side

Na = 1

O = 1

H = 2

Cl = 1

Since equal number of atoms of each element are present at both the reactants and products sides, then the equation is balanced

  • For 2NaOH + 2HCl → 2NaCl + H2O

On the reactants side

Na = 2

O = 2

H = 4

Cl = 1

On the products side

Na = 2

O = 1

H = 2

Cl = 2

The number of atoms of each element present at the reactants side and that present at products side are not equal.

∴ The equation is not balanced

  • For 2NaOH + HCl → NaCl + H2O

On the reactants side

Na = 2

O = 2

H = 3

Cl = 1

On the products side

Na = 1

O = 1

H = 2

Cl = 1

The number of atoms of each element present at the reactants side and that present at products side are not equal.

∴ The equation is not balanced

  • For NaOH + 2HCl → NaCl + H2O

On the reactants side

Na = 1

O = 1

H = 2

Cl = 2

On the products side

Na = 1

O = 1

H = 2

Cl = 1

The number of atoms of each element present at the reactants side and that present at products side are not equal.

∴ The equation is not balanced

Hence, the equation that is balanced is the first equation NaOH + HCl → NaCl + H2O

Learn more here: brainly.com/question/17015942

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