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Thread: Quick PH Question

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    Beast Zoinked's Avatar
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    Quick PH Question

    2007 HSC Exam 21b) (No sample answers for some reason)

    Using the red cabbage indicator, what colour would the solution be if 10 mL of
    0.005 mol L–1 H2SO4 was diluted to 100 mL?

    I did 0.005 x 2 x 0.1 = 0.001. Then -Log10(0.001) = Ph of 3 and that the indicator would turn violet.

    Is that correctly done?

    https://www.boardofstudies.nsw.edu.a...emistry_07.pdf

    2016 ATAR: >90
    2017-2020: Bachelor of Applied Finance/Bachelor of Commerce (Accounting) @ Mq Uni

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    Ancient Orator leehuan's Avatar
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    Re: Quick PH Question

    a)
    pH = -log10(0.005*2)=2

    This is enough information to say that your answer to part b) is a correct one.

    (Naturally, we are assuming full ionisation of both protons of sulfuric acid, when in reality only one definitely ionises and the other just ionises noticeably easily)

    Reason: A dilution by a factor of 10 is equivalent to decreasing the concentration 10-fold. A decrease in the concentration of the acid 10-fold also decreases the concentration of the hydronium ion by that much.

    If the concentration of the hydronium ion is decreased 10-fold, we precisely increase the pH by one. This is because pH=-log10[H3OO+] is essentially a base 10 logarithmic scale.

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    Beast Zoinked's Avatar
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    Re: Quick PH Question

    Thanks leehuan, knew I could count on you

    2016 ATAR: >90
    2017-2020: Bachelor of Applied Finance/Bachelor of Commerce (Accounting) @ Mq Uni

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