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Science Question Answer (1 Marks)

Atoms and Molecules · Maharashtra Board STD 9 (MSBSHSE - English Medium). 75 questions.

Questions · Page 2 of 2

Question Answer (1 Marks)

Question 511 Mark
Work out the formulae for the following compounds:
Calcium carbonate.
Answer
Calcium carbonate:$\ \text{Symbols}:\ \ \ \ \ \text{Ca}\ \ \ \ \ \ \ \ \ \text{CO}_3\\ {\text{Valencies}}:\ \ +2\ \ \ \ \ \ \ \ -2$
Thus, the resulting compound is $CaCO_3$
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Question 541 Mark
Name the element used as a standard for atomic mass scale.
Answer
Carbon is used as a standard for atomic mass scale.
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Question 551 Mark
An element $Y$ has a valency of $4$. Write the formula for its:
  1. Chloride.
  2. Oxide.
  3. Sulphate.
  4. Carbonate.
  5. Nitrate.
Answer
  1. $YCl_4$
  2. $YO_0$
  3. $Y(SO_4)_2$
  4. $Y(CO_3)_2$
  5. $Y(NO_3)_4$
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Question 581 Mark
An element $Z$ has a valency of $3$. What is the formula of oxide of $Z$?
Answer
Valency of element Z = $3$
Valency of oxygen = $2$
So, formula of oxide of element = $Z_2O_3$
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Question 601 Mark
An element $X$ has a valency of $2.$ Write the simplest formula for:
Bromide of the element.
Answer
Bromide of element:
As valency of bromine is $-1$ and that of element $X$ is $+2$ so, the resulting compound is $XBr_2$.
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Question 611 Mark
What is meant by $‘a$ mole of carbon atoms’?
Answer
A mole of carbon atoms means a carbon sample weighing $12\ g$ and containing $6.022 \times 10^{23}$​​​​​​​ carbon atoms.
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Question 621 Mark
Calculate the molecular mass of chloroform $\left( CHCl _3\right)$.
(Atomic masses: $C =12 u ; H =1 u ; Cl =35.5 u$ )
Answer
Molecular mass of chloroform $\left( CHCl _3\right)$
$=1 \times C+1 \times H+3 \times Cl$
$=(12+1+106.5) u$
$=119.5 u$
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Question 631 Mark
Dalton’s atomic theory says that atoms are indivisible. Is this statement still valid? Give reasons for your answer.
Answer
No, the statement is not valid because atoms can be divided into subatomic particles called electrons, protons and neutrons.
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Question 641 Mark
Write the symbols/ formulae of two simple ions and two compound ions (or polyatomic ions).
Answer
Write the symbols/ formulae of two simple ions and two compound ions (or polyatomic ions).
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Question 651 Mark
Name the particle which has $18$ electrons, $18$ neutrons and $17$ protons in it.
Answer
Its $Cl^-$, the chloride ion.
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Question 681 Mark
Name the law of chemical combination:
Which was given by Proust.
Answer
The law of chemical combination proposed by Proust is well known as the law of constant proportions.
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Question 691 Mark
Define atomic mass unit. What is its symbol?
Answer
One atomic mass unit is defined as exactly one-twelfth the mass of an atom of carbon-12. Its symbol is 'u'.
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Question 701 Mark
What is meant by saying that ‘the atomic mass of oxygen is 16’?
Answer
It means that one atom of oxygen is 16 times heavier than 1/ 12 of a C-12 atom.
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Question 711 Mark
The radius of an oxygen atom is 0.073nm. What does the symbol ‘nm’ represent?
Answer
The symbol 'nm' in 0.073nm (radius of an oxygen atom) represents the unit of atomic radius, i.e. nanometre.
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Question 721 Mark
Write down the formulae for the following compounds:
Magnesium hydroxide
Answer
Magnesium hydroxide: $Mg(OH)_2$
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Question 731 Mark
Calculate the formulae masses of the following compounds:
Aluminium oxide, $Al_2O_3$
(Given: Atomic masses: $Na = 23\ u; O = 16\ u; Al = 27\ u$)
Answer
Molecular Mass of $Al_2O_3 = (2 \times Al) + (3 \times O)$
$= (2 \times 27) + (3 \times 16)$
$= 102u$
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Question 741 Mark
Calculate the molecular mass of nitric acid, $HNO_3$​​​​​​​.
(Atomic masses: $H = 1u; N = 14u; O = 16u$)
Answer
Molecular mass of Nitric acid $(HNO_3)$
$= 1 \times H + 1 \times N + 3 \times O$
$= (1 + 14 + 48)u$
$= 63\ u$
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Question 751 Mark
What is the difference between $2H$ and $H_2$​​​​​​​?
Answer
$2H$ represents two separate atoms of hydrogen and $H_2$​​​​​​​ represents one molecule of hydrogen.
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Question Answer (1 Marks) - Page 2 - Science STD 9 Questions - Vidyadip