# Chemistry Class 11 NCERT Solutions: Chapter 1 Some Basic Concepts of Chemistry Part 5

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Q: 13. Pressure is determined as force per unit area of the surface. The SI unit of pressure, Pascal is as shown below:

If mass of air at sea level *is**,* calculate the pressure in Pascal.

Answer:

Pressure is defined as force acting per unit, area of the surface.

We know,

Then,

∴

Q: 14. What is the SI unit of mass? How is it defined?

Answer:

The SI unit of mass is kilogram (kg). 1 kilogram is defined as the mass equal to the mass of the international prototype of kilogram.

Q: 15. Match the following prefixes with their multiples:

Prefixes | Multiples | |

(i) | micro | |

(ii) | Deca | |

(iii) | Mega | |

(iv) | Giga | |

(v) | Femto |

Answer:

Prefix | Multiples | |

(i) | micro | |

(ii) | Deca | |

(iii) | Mega | |

(iv) | Giga | |

(v) | Femto |

Q: 16. What do you mean by significant figures?

Answer;

Significant figures are those meaningful digits that are known with certainty. They indicate uncertainty in an experiment or calculated value. For example, if mL is the result of an experiment, then 15 is certain while 6 is uncertain, and the total number of significant figures are 3.

Hence, significant figures are defined as the total number of digits in a number including the last digit that last digit that represents the uncertainty of the result.